Darrin, obviously. Dick York had to be replaced because of a past disabling back injury that had torn the muscles on the right side of his back. During his final season on Bewitched, entire episodes had him sitting on a couch or lying in bed. The pain had become so bad at that point, he bit a hole into his tongue. The experience prompted him to quit the show. York would go on to spend the rest of his life bed-ridden with an addiction to pain killers.
History repeated itself 15 years later when the actor who played the father on another supernatural sitcom, the 1980s obscurity Down to Earth, was replaced. The replacement: Dick Sargent!
Gladys Kravitz was originally played by Alice Pearce. When she died of ovarian cancer in the middle of the second season, Sandra Gould replaced her (the following season — in the interim, Abner's sister, Harriet Kravitz, played by Mary Grace Canfield, "filled in" for Gladys).
Louise Tate (Larry's wife) was first played by Irene Vernon, later by Kasey Rogers.
Frank Stephens, Durwood — er, Darrin's father; first played by Robert F. Simon, later by Roy Roberts (with an occasional reprise by Simon).
In fact, the changes to Gladys Kravitz, Louise Tate and Frank Stephens, as well as Erin Murphy assuming the role of Tabitha, all occurred between the second and third seasons, when the show switched from black-and-white to color, only compounding the show's different look and feel.
The Ur Example is The Goldbergs (not to be confused with the unrelated ongoing series of the same name). The television version ran on CBS Television from 1949 to 1951 and co-starred Philip Loeb as Jake Goldberg. In 1950, Philip Loeb was blacklisted and accused of being a Communist, and pressure was placed on Gertude Berg (who both starred and owned the television version as she had the radio original) to fire him. When she refused, General Foods cancelled their sponsorship, and CBS dropped it from their schedule by June 1951. Eight months later, however NBC—the show's original broadcasting home—picked up the series for the 1952–53 season, but informed Gertrude Berg that if she persisted in allowing Philip Loeb to remain with the series, it would never be seen on television again. She finally gave in, and the series reappeared in a twice-weekly, early-evening 15 minute format (with another change in title, to Molly, in due course), with Harold Stone and then Robert H. Harris replacing Loeb as Jake, though Berg quietly continued to pay a salary to Loeb.
Mad About You played this trope straight, then subverted it, in the form of Paul and Jamie's neighbors Maggie and Hal Conway. Paxton Whitehead originated the role of husband Hal Conway and played it for a few episodes. When he wasn't available for an episode, actor Jim Piddock took over the role, implicitly playing the same Hal. Much later, Whitehead was available again. In the first episode upon his return Maggie says, "I divorced my second Hal, and remarried the first one."
The trope was played completely straight with Jamie's parents, who are played by three sets of actors, culminating in the Stunt Casting of Carol Burnett and Carroll O'Connor.
Roseanne had lots of fun with this trope when Sarah Chalke replaced Lecy Goranson as the Conners' eldest daughter Becky for three seasons. The scriptwriters brazenly point it out at several points, even after the original actress returned.
When the new Becky angers Roseanne in one episode, Roseanne quips, "I can have you replaced, you know."
The very first time the original Becky appears onscreen upon returning this conversation ensues between her and Roseanne.
Becky: *carrying paint rags* Here, dad this is all I could find. Roseanne: Where in the hell have you been? Becky: Don't yell at me mother, I was getting this. Roseanne: Well it took you long enough seems you've been gone for three years.
Darlene's first words to the character are "Where the hell have you been?". Which she replies by asking "why does everyone keep saying that to me?" during which the actress is barely holding back laughter.
In one episode, both actresses appeared, recreating the opening sequence from The Patty Duke Show (although some thought it was the Marx Brothers' famous "mirror gag"). The second Becky appears at the Conner's doorstep with a pair of trick or treaters and cheerful attitude, Mark (the character's husband) remarks "this is like Deja'vu all over again" and Roseanne later comments "Gee, I wish we had a daughter that sweet."
Chalke was introduced by the cast specifically referencing this trope — in The Tag of her first episode, the family is watching Bewitched and the topic of how an actor change should be presented to the viewing audience is brought up, right before the camera focuses on the new Becky for the first time. She then smilingly comments that she prefers the second Darrin.
Also, in the Disney vacation episode, the Becky actresses changed AGAIN for that one episode, and when the actress first entered the episode it paused, a voiceover explained the full history of the two actresses switching, and later Roseanne comments "Aren't you happy you were here today?"
Also, a Clip Show with a framing sequence set in the future featured John Goodman as an adult DJ, almost catatonic following some mysterious trauma in his childhood. He just kept repeating "They say she's the same, but she isn't the same."
...and Becky wasn't even the only Conner child to be replaced. Sal Barone, who played DJ only in the pilot, was replaced for the series by Michael Fishman.
In a take similar to Roseanne, the characters in Bette Midler's sitcom Bette were discussing this trope around the breakfast. Bette says something like "Don't you hate that, Roy?" Roy looks up from behind the fridge door, and we see that he's now played by Robert Hays instead of Kevin Dunn.
In the second episode of Friends, Ross's ex-wife Carol was played by Anita Barone, whose look and acting style were nothing like those of Jane Sibbett, who replaced her for the rest of the series.
Also Rachel's friend Mindy, who eventually married her ex-fiancé, was originally played by Jennifer Grey.
Happens to Ben and Emma too, but it's less noticeable since they were both babies when this happened.
A year later, Barone played a major role on The Jeff Foxworthy Show for an entire season, then Ann Cusack replaced her.
Don Draper's son Bobby on Mad Men. As of the 5th season, the new actor will be the Other Other Other Darrin.
The 1980's British sitcom Bread replaced actors for three of its major characters. Julie (Billy's girlfriend) was played by Caroline Milmoe for the first season and Hilary Crowson from season 2 to 5. Joey was played by Peter Howitt from season 1 to 5 and Graham Bickley thereafter. Aveline was played by Gilly Corman from season 1 to 4 and Melanie Hill thereafter.
Degrassi Junior High quietly got rid of a wheelchair bound actress and replaced her with another girl in a wheelchair, most likely when the producers decided to actually make her into a significant character. The Agony Booth recap of a first season episode acknowledges this, along with the trope namer, referring to the first actress as "the Dick York of Wheelchair Girls."
The character of Elizabeth Weir was introduced in the two-part seventh season finale of Stargate SG-1, played by Jessica Steen. When she reappeared in the first two episodes of Season 8, and then as the lead in the spin-off Stargate Atlantis, she was played by Torri Higginson. Jessica Steen was reportedly rather disappointed that she wasn't even asked to reprise the role. This was a rather jarring replacement, as Jessica Steen (at the time) was blonde, and Torri Higginson is a rather dark brunette. A case where they didn't bother to at at least cast a similar looking actress.
For the character's final, send-off episode, Weir was played by yet another actress, Michelle Morgan (with a fourth actress portraying her in flashbacks!), although the switch was justified in-plot by stating that Weir had downloaded her consciousness into a series of android bodies after her original body was destroyed by the Replicators.
Also, the character of Cassandra was originally played by Katie Stuart in 'Singularity' but was later played by Colleen Rennison in 'Rite of Passage'.
...because Katie Stuart was busy being the Other Kitty Pryde in X2.
The humanesque wraith Michael was played by Connor Trinneer in all but one appearance, where he was played by Brent Stait. Connor did, however, still provide the character's voice.
The first time we meet Teal'c's wife, Drey'auc, she is played by Salli Richardson. Subsequent appearances would be by Brook Parker.
Ben, the older of the two Hartman brothers on Hearts Afire, was recast in the show's third season. He changed from a stocky blonde to a skinny brunette and didn't bear any resemblance whatsoever to his previous actor or his character's brother.
In a first-season episode of The Facts of Life, Blair's mother was played by a voluptuous early-30s blonde actress in what was apparently a one-shot appearance. However, several seasons later, Blair's mother became a Recurrer, and she was played by svelte 40-something brunette.
On Sabrina the Teenage Witch, Sabrina's mother was played by a different actress each of the (very few) times she appeared. Her father, who appeared far more often, also switched actors between visits.
On Golden Girls, different actresses play visiting children quite a lot. Rose's daughter Kirsten was first played by Christina Belford in her initial appearance, then later played by Lee Garlington. Dorothy's daughter Kate was first played by Lisa Jane Persky, then by Deena Freeman. Dorothy's sister Gloria was first played by Doris Belack, then by Dena Dietrich. The most drastic difference was Blanche's daughter, who was played by a heavy actress in her first appearance, then by a much slimmer, more attractive actress in her second appearance.
On The Partridge Family, the first Chris, Jeremy Gelbwaks, was fired at the end of the first season (reportedly for starting fights with other cast members) and replaced by Brian Forster, who looked absolutely nothing like him.
Actress Caitlin Brown played the Narn attache Na'Toth in the first season. Brown then quit due to trouble with her prosthetic makeup and also in order to expand her film career - she was replaced by Mary Kay Adams. In one episode of the final season, Na'Toth reappeared alive after being presumed dead, in a Centauri prison cell - Caitlin Brown came back for this final appearance of the character.
Also, the first actor to play the recurring role of Draal was unavailable after one episode, so a younger actor was selected with a Lampshade Hanging that the machine he was plugged into in his last appearance not only extended his life but restored his youth. Later the second actor left for Broadway, and the character was dropped entirely as everyone agreed it would be too much to expect audiences to accept a third actor. To further complicate matters, the original actor later returned portraying a different recurring character, Brother Theo.
Sheridan's wife Anna was originally portrayed in photos, video clips and flashbacks by Beth Toussaint, who looked nothing like Melissa Gilbert, brought in when Anna became a "live" on-screen character after her "resurrection". At the time of her casting, producer J. Michael Straczynski considered re-editing earlier episodes to replace the original actress with Melissa Gilbert but decided against it.
On the 1990s Superman TV show, Lois and Clark, the character of Jimmy Olsen was played by Michael Landis in Season 1, but was then replaced by Justin Whalin in Season 2. The showrunners were worried that Landis looked too much like Dean Cain and played Jimmy is too "slick" rather than the slightly geeky character they wanted.
Lois and Clark also had the distinction of recasting Lois' entire immediate family. Lois's father was initially played by Denis Arndt, providing a bland Sam Lane in the character's Season 1 introduction, and Harve Presnell as a more caustic version in a string of appearances starting in Season 3. Her sister, Lucy, was initially portrayed by Elizabeth Barondess in the show's first three episodes before disappearing; when she returned, she was played by Roxana Zal. Lois's mother was played by Phyllis Coates during the first season finale, and in subsequent appearances by Beverly Garland. Coates' appearance on Lois and Clark also qualified as Stunt Casting, as she played Lois Lane in the feature film "Superman and the Mole Men" and later in the classic The Adventures of Superman TV show (where, ironically, she was replaced after one season by Noel Neill, who had previously played Lois in Superman's first live-action screen appearances, the 1948 and 1950 serials starring Kirk Alyn — making Neill her own Other Darrin's Other Darrin).
Clark's biological father, Jor-El, was played by three different actors: David Warner in season 1's "Foundling", an unknown in season 3's "Never on Sunday" and François Giroday in the season 3 finale "Big Girls Don't Cry". All portrayals were flashbacks or recordings from when Clark was a baby on Krypton, so the three actors were portraying the same character within a relatively short timeframe.
Lois and Clark featured several appearances by a time-traveling H.G. Wells (an idea that probably deserves its own trope page), but due to actor availability issues, his appearances alternated between an older Wells played by Hamilton Camp and a younger one played by Terry Kiser.
Also happened on another Superman show, The Adventures of Superboy, except this time, it was the title character. Superboy actor John Haymes Newton supposedly demanded a higher salary and/or was convicted of a DUI after the first season of the show, so he was replaced with Gerard Christopher for Season 2.
The same thing happened with the role of Lex Luthor, which passed from Season 1's Scott Wells to Sherman Howard for the remainder of the show.
Gordon on Sesame Street has been played by three actors: Matt Robinson, Hal Miller, and (currently, since 1974) Roscoe Orman. Orman has become the definitive Gordon; in fact, many people don't even know that anybody but Orman ever played the character.
In the first pilot episode, Gordon was also played by Garrett Saunders. Saunders was unknown until late 2011, as part of a campaign to identify him. 
Strangest example ever: the two Gabys from Ghostwriter. Gaby started out as Alex's pesky little sister, until the actress hit her growth spurt and was as tall as Alex. So later scripts changed Gaby's personality to reflect this. Then the actress was replaced by a new, shorter one — and Gaby reverted to her old personality.
Though Doctor Who usually uses The Nth Doctor, having an in-story use for the changed incarnations, there were a few incarnations that had Other Darrins. For the "The Five Doctors", which featured the first five Doctors (albeit one in usused archive footage), William Hartnell, the actor who originally played the First Doctor, had already since passed away, so Richard Hurndall stepped into the part.
Davros, creator of the Daleks, was played by Michael Wisher in Season 12, then in his next appearance in Season 17 by David Gooderson, then throughout the 80s and later in the Big Finish Doctor Who and I Davros audios by Terry Molloy. When he reappeared in series 4, he was played by Julian Bleach. While the heavy mask prosthetics makes it hard to tell (aside from the mask made for Wisher fitting horribly on Gooderson), their voices do all differ from each other, despite Sarah Jane recognising the voice of Bleach's Davros after meeting Wisher's.
Colin Baker, the Sixth Doctor, was notable for having been fired from the job. Since Colin wasn't willing to return only for a regeneration scene, the next season began with Sylvester McCoy laying down on the ground, in Sixth Doctor costume and a wig, until the regeneration into the Seventh incarnation moments after the beginning of the episode. (Given the circumstances, this would count as a Bridge Drop, too.)
Peter Pratt and Geoffrey Beevers played the same incarnation of the Master in different stories. As with Davros, the heavy makeup may have made it hard to tell. According to the Expanded Universe, this is supposed to be a heavily decayed version of (sans heavy makeup) Roger Delgado's incarnation, who was a semi-regular from 1971-73.
In the Second Doctor story The Macra Terror, Chicki was played by different actresses in Part 1 and Part 4.
Game On replaced the actor playing Matt, Ben Chaplin, with Neil Stuke, and concluded Stuke's first episode with the characters watching Roseanne commenting how much they hated it when a show switched actors. (Almost certainly a Shout-Out to Roseanne doing a Shout-Out in The Other Becky's first episode, with Bewitched.)
GOB's girlfriend Marta on Arrested Development was played by three different actresses during the series' run. This is pointed out in the DVD commentaries. George Michael's girlfriend Ann was also played by a different actress in her first appearance.
Word of God is that they planned to cast a new actress for Ann Veal for each appearance; this was abandoned but the "Her?" Running Gag remained.
Bobby's dead wife in Supernatural, whose original actress from the third season episode "Dream A Little Dream of Me" was pregnant at the time of her reappearance in Season 5. The producers seemed uncomfortable with the idea of a pregnant zombie wife.
On Titus, Titus's violent, manic-depressive schizophrenic mother Juanita was played by three different actresses. In season one's "Mom's Not Nuts," Juanita was played by Christine Estebrook. In season two's "Locking Up Mom," "The Reconciliation," and "The Wedding," Juanita was played by Frances Fisher. In season three's "The Trial" and "The Visit," Juanita was played by Connie Stevens note It should be noted that Christine Estebrook had Juanita as a brunette, while Fisher and Stevens had Juanita as a redhead. Whether this reflected Juanita's mental illness or was merely an oversight in casting is not known.
Krista Errickson replaced Donna Wilkes as Diane Alder after the first season of Hello Larry.
In the Disney series Flash Forward, Becca's sister Ellen was played by Rachel Blanchard for the first four episodes and Robin Brule for the rest of the series. It should be noted, however, that the amount of real-world time between episodes four and five allowed Jewel Staite to film the entire first season of Space Cases.
Played with in Due South, when Callum Keith Rennie was brought on to replace David Marciano, the original Ray Vecchio: Rennie appeared in the switchover episode as "Vecchio", who turns out in fact to be an entirely new character, Ray Kowalski, imported by the FBI to impersonate Vecchio while he went undercover with the Mob despite looking nothing like Vecchio. Benton Fraser, the main character, spends most of the episode being the Only Sane Man, who alone appears to have noticed the change as the supporting cast all insist that this is indeed Vecchio, before finally getting round to putting him in the picture. (Both Rays put in an appearance in the series finale.)
But played straight with Fraser's deaf half-wolf Diefenbaker: played in the Pilot Movie by Newman, the first & second seasons by Lincoln, and Draco in the third.
Bonnie Dennison took over the role of Emily Yokas for Third Watch's fourth season, as the character became more involved in the storylines. This caused many characters to question when Emily "became so grown up".
On Profiler, Sam Waters' daughter, Chloe, played for two seasons by the prepubescent Caitlin Wachs, suddenly turned into the obviously adolescent Evan Rachel Wood in Season 3. Thus, a character who - within the show's time frame — was sleeping with stuffed animals and being read bedtime stories a week ago became concerned with boys and make-up almost literally overnight. Kids grow up so fast, don't they?
Vice-president's brother and living supposed murder victim Terence Steadman from Prison Break was first played by John Billingsley, and then Jeff Perry, due to Billingsley becoming a regular on another show. Ironically, he's kidnapped by the main characters who argue over whether he'll be seen as just a imposter.
Seinfeld handled this interestingly. The first time George's father appeared, he was played by John Randolph, who was replaced by Jerry Stiller for all later appearances. The scenes with John Randolph were actually re-shot with Jerry Stiller playing the part, and the re-shot scenes were what was shown in reruns.
Similarly, the character of Newman made his debut with a voice-only appearance (played by creator Larry David) in Season 2's "The Revenge". After Wayne Knight assumed the role on-screen, Newman's lines in "The Revenge" were rerecorded with Knight's voice.
Jerry's father was played by Phil Bruns in his first appearance and Barney Martin subsequently; the producers had wanted to reshoot with Barney Martin too, but found that the other actors had aged too much since the first season for the replacement to be convincing.
The character of Lloyd Braun was played by Peter Keleghan in his first appearance and Matt McCoy in subsequent appearances. The two actors look nothing alike.
In Curb Your Enthusiasm, Larry David makes a Seinfeld reunion. When Jason Alexander quits, Larry suggests that he will play George instead, and brings up the other Darrin as an example.
The meta-humor here is that "George" was originally created a stand-in for Larry David's personality; David did not feel confident acting in a sitcom.
Frasier Crane's first wife, Nanette (aka children's entertainer Nanny G) appeared in one episode of Cheers and two episodes of Frasier, and was played by a different actress each time.
The dog Eddie was originally played by Moose; over the course of the later seasons, Moose was replaced by his son Enzo.
Passions parodied the use of voice-over to announce a casting change when Mark Cameron Wystrach took over the role of Fox Crane in February 2006. The voice-over played out, and Tabitha, who was in the scene looked for the source of the voice-over, before it scolded her to open the door and meet the new Fox.
In The Fresh Prince of Bel-Air, Vivian's actress - Janet Hubert-Whitten - was replaced in the third season, due to complications with a pregnancy. She was replaced with Daphne Maxwell Reid who remained for the rest of the series. This was lampshaded at the beginning of the 5th season, when Jazz asked Will Smith who they got to replace Vivian this time and a Soap Opera Rapid Aging Syndrome-addled Nicky Banks answered "It's the same mom!"
In fact it was lampshaded quite openly numerous times throughout the show. Will Smith often mugged for the camera after making these comments, which were generally ad-libbed.
Lampshaded in The Fast Show's Live Episode a recurring character was played some another actor and aged to 18 (presumably so they didn't have to deal with children in a live performance), and is greeted with "18, eh? You look so different, I hardly recognize you. Almost like a different actor!"
In Blake's 7 Travis (The Dragon) is played by Stephen Greif in Season 1 and Brian Croucher in Season 2. The very minor character Ven Glynd has two appearances and is played by a different actor each time. The voice of Orac was provided by Peter Tuddenham, who also voiced the Liberator computer Zen, after original Orac voice artist Derek Farr was unwilling to sign up as a regular cast member.
In The Munsters, Marilyn Munster was played by two different actresses: Beverley Owen (in the first 13 episodes) and Pat Priest (in the rest of the series). Marilyn was recast yet again for the follow-up theatrical release, Munsters Go Home.
Somewhat odd example in Monk: Dale "the Whale" is played by three different actors in his three appearances: Adam Arkin in "Mr. Monk Meets Dale the Whale," Tim Curry in "Mr. Monk Goes to Jail," and Ray Porter in "Mr. Monk Is On The Run, Part 2". All three did the role in fatsuits and, under their makeup and prosthetic fat, looked quite similar and could have probably been passed off without many people noticing- except that the second actor was Tim Curry, who made no attempt to disguise his trademark accent.
Porter seems to have done away with the accent.
Four different actresses played Trudy Monk. Stella Rusich played Trudy in the character's first four appearances, Melora Hardin then played Trudy for an additional ten appearances, Rose Abdoo was Trudy for one episode between Hardin's fourth and fifth appearances ("Mr. Monk and Little Monk"), and Lindy Newton played Trudy in the college flashback episode "Mr. Monk and the Class Reunion".
Benjy, Sharona's son, was played by Kane Ritchotte in the pilot, season 2 and the first half of season 3, and by Max Morrow in season 1 (except for the pilot). This was because the pilot was produced in Vancouver, on the West Coast (where Ritchote lived), and the regular season 1 episodes were filmed in Toronto, Ontario (where Morrow lived).
When Josh Weinstein left after one season of Mystery Science Theater 3000, Jim Mallon took over as the voice of Gypsy. It's possible that most people never noticed since Gypsy had very few lines, and was close to being The Unintelligible when she did speak. Likewise, Kevin Murphy took over as the voice of Tom Servo; but this was subverted since the difference was explained in-series as Joel tinkering around with things.
The departure of Trace Beaulieu (who also played Dr. Clayton Forrester) and Mallon when the show moved from Comedy Central to Sci-Fi resulted in changes to the voices of Crow T. Robot and Gypsy, respectively. Crow's voice change was alluded to in the Sci-Fi opening credits, during which Crow proclaims, "I'm different!"
In a later episode, Crow is telling Mike (who's turning into a Were-Crow) what to expect from his transformation, and mentions that "Your voice is gonna change inexplicably every seven years or so."
In season premiere of season ten, Joel takes notice of Crow's different voice and says that Crow "changed his beak".
Two of Jeeves and Wooster's characters are played by three different actors each (Madeline Bassett and Constable Oates) and Aunt Dahlia is played by a different actress every series. The total number of characters (some fairly major) who change actors at some point runs into double figures. This can result in a fair amount of confusion and arguments when watching the entire run, due to the show making no real effort to ease the transitions for the viewer. Not to mention having an established character suddenly become Martin Clunes is a little jarring...
This is made much worse by the fact that some of these actors later appear as different characters - for example, one actress who started out as Madeline Bassett plays Florence Craye in later episodes.
In My Wife and Kids Claire was played by Jazz Raycole in the first season. Jennifer Freeman (who looks nothing like Jazz Raycole) plays her in the second onwards, when Freeman debuted as Claire her father Michael (Damon Wayans) even commented saying "I don't know what it is, but you look like a whole new person!". The reason was because Raycole's mother objected to the then upcoming "Claire's friend is pregnant" plot and pulled her daughter out of the show. Junior's girlfriend Vanessa was also played by two actresses (Meagan Good then Brooklyn Sudano).
YATR is not unique in this respect. Due to the fact American daytime soap operas have a tendency to feature the same characters for decades, it's not uncommon for roles to be recast on a temporary or permanent basis when an actor decides to go on to do something else, but the writers have more stories to tell using the character.
Shatner's predecessor in Star Trek: The Original Series, Jeffery Hunter, played Captain Pike in "The Cage". This footage was later re-used in "The Menagerie", with Pike himself appearing a motionless deformity in an iron lung-type device. This was primarily to disguise the fact that Hunter was now a film star and thus unavailable; this new Pike was played by a lookalike (such as he is) named Sean Kenney. What's interesting is that Captain Pike was retconned into Kirk's predecessor, as well; He was the original Captain of the Enterprise, with Mr. Spock as his first officer. This is still canon in the Abrams film, in which Bruce Greenwood plays Pike.
Tora Ziyal was portrayed by three different actresses as the show went on: Cyia Batten in her first two appearances, Tracy Middendorf for one episode, and Melanie Smith for the rest of the series. The character was 19 years old when first introduced, but the producers decided to age up the actress (despite the original actress already being older than her character) to emphasise the character's maturity in preparation for developing a relationship plotline with an older male character to avoid any perception of cradle-robbing occuring (by the time the relationship was established, Ziyal was at least 21 years old and being played by a 34-year old actress).
Quark's mother was played by two different actresses (SCTV's Andrea Martin in her first appearance, and Cecily Adams, daughter of Don Adams, in all subsequent appearances), due to the original actress not being able to bear the prosthetics, but due to the heavy makeup and low frequency of her appearances, it's less noticeable.
Neral, the Romulan leader, was played by Norman Large in an episode of Star Trek: The Next Generation, then by the noticeably older and thinner Hal Landon Jr. in an episode of Deep Space Nine. Lampshaded somewhat by the official card game, where Neral's card states he "has not aged well under the burden of leadership."
Naomi was first played by Brooke Stephens and then by Scarlett Pomers for the rest of the series.
The Borg queen was played by Alice Krige in Star Trek: First Contact and the final episode of Voyager. In three episodes of Voyager between the aforementioned movie and final episode, she was played by Susanna Thompson.
In a Season 2 episode of Voyager, Tom Paris, under the influence of a telepathic alien, has a vision of his father, played by Warren Munson. When we meet the elder Paris in Season 6 (on Earth, working on a way to bring the titular ship home), he is played by Richard Herd, who continues to portray the character through the series finale.
Similarly the character of warp drive inventor Dr. Zefram Cochrane, who was played by Glenn Corbett in the Star Trek: The Original Series episode "Metamophosis" is played here by James Cromwell. (Corbett had already passed away by the time the movie was shot).
The UK sitcom All About Me replaced its star Meera Syal with Nina Wadia after one series, as Syal became more famous and moved on to bigger and better.
Winning quite possibly the award for the strangest example of all time- Christina Crawford, appearing in The Secret Storm from 1968-1969 got an ovarian tumour. While she was off the show, they got a stand-in. Joan Crawford- her adoptive mother- and more than thirty years older than her.
Iconic Australian SitcomHey Dad..! did it with not one, but two characters. Simon Kelly was played by Paul Smith in 1987, then by Christopher Mayer from 1988-91. Simon's little sister Jenny was played by Sarah Monahan from 1987-93 and by Angela Keep from 1993-94. Jenny's change was lampshaded with a line like "You've really changed at that boarding school", but Simon's change was completely unacknowledged.
The Worst Witch featured three such changes. The first one saw Felicity Jones replaced as Ethel Hallow by Katy Allen for Seasons 2 and 3, which was explained away as a "makeover". Jones later returned for the role in the fourth season (Weirdsister College). Fenella Feverfew was played by Julia Malewski for the first season and Emily Stride for the third. The character of Miss Hardbroom was also played by two different actresses: Kate Duchene in the first three seasons, and Caroline O'Neill in The New Worst Witch. In fact, only Georgina Sherrington (Mildred) has appeared in all three incarnations of the show.
The child swap on Psych; Josh Hayden, the child actor playing Young Shawn, appeared only in the pilot. In other appearances, the part is played by Liam James and Skyler Gisondo.
They did it with Young Gus, too. Carlos McCullers II replaced Isaah Brown after 5 episodes.
And Gus' father, who has been played by Ernie Hudson and Keith David.
In the second episode of Quantum Leap, it shows Sam's father as a separate actor. In a later episode when Sam leaps into a younger version of himself, his father is also played by Scott Bakula, just in make up to look older.
Clare Buckfield replaced Georgina Cates as Jenny after the first series of 2point4 Children
Boy Meets World combined Chuck Cunningham Syndrome with Suspiciously Similar Substitute; Corey's younger sister Morgan was played by two child actresses, in non-consecutive seasons. When the new Morgan appeared, she said she had been sent to her room, and was only now coming back down. Lampshaded when she mentioned in a dazed tone that she had been up there "a long time."
Also occurs to Topanga's parents, where they are almost completely different characters whenever they appear. Topanga's father is played by Peter Tork of The Monkees during the second/third season guest appearances, by Michael McKean in Season 6 and then by dark-haired Mark Harelik in the last season. While Peter Tork accentuated the character's hippie tendencies, the replacement actors made him more "normal". Her mother also changes actresses despite only appearing towards the end of the show, played by Anette O'Toole in the Season 6 finale and by Marcia Cross in the Season 7 appearances.
Laurie from That '70s Show was played by Lisa Robin Kelly in Seasons 1-5 and replaced by Christina Moore (who was one of many short-lived MADtv cast members in the same vein as Simon Helberg and Taran Killam) in the 6th season.
Also of note is Jackie Burkhart's mother Pam was originally portrayed by Eve Plumb but when she returned from an extended vacation Brooke Shields had replaced her.
On Everybody Loves Raymond, in the Season 4 episode "Hackidu", Amy's brother Russel owned a comic store and was played by Paul Reubens. In the Season 7 episode "Just a Formality", we meet Amy's brother again; not only is he now played by Chris Elliott, but his name has been changed to Peter. However, he still remembers Raymond coming to his comic store. He becomes a recurring character for the rest of the series.
On Charmed, the sisters' father appears for one episode in the first season, played by Tony Denison. In all subsequent appearances of the character, he was played by James Read, who looks nothing like the original actor. The change was never noted by the characters or explained.
The actress playing Harriet was changed on the very last minute for Family Matters' final season. The original actor felt that the show was suffering Seasonal Rot and wanted no more part of it.
On The O.C., Ryan's brother Trey switched actors from the second season onward, with no fanfare.
Marissa's sister Kaitlin is played by two different actresses, with the switch coming after the character returns from boarding school. A few characters remark on how much she's changed. (Seth: "Mini-Coop ... not so mini.")
My Big Fat Greek Life, the TV sequel to the film My Big Fat Greek Wedding, managed to snare every major actor from the film except for John Corbett, who had played the groom in said wedding. The series begins with his wife's mother telling him "You look different", then quickly commenting that her daughter does too.
Chloe Annett replaced Clare Grogan as Kochanski in Red Dwarf when she became a main character. Might be justified by her being from a parallel universe, but Lister reacts as if she's just like "his" Kochanski.
After a single episode as Kryten in series 2, David Ross was replaced by Robert Llewellyn when Kryten permanently joined the crew in season 3.
The character Gary Ewing first appeared on a two-part episode of Dallas played by David Ackroyd, who was replaced by the completely different looking Ted Shackleford when the character spun off to Knots Landing the following year. His wife Val, who had appeared in a few more episodes and was therefore more recognizable, was not replaced.
Dallas also set up one of these for Pam Ewing, as after Victoria Principal left the show the character supposedly died in a car crash, but later appeared with her head covered in bandages after extensive reconstructive surgery. However, she never did appear again.
There's also Donna Reed taking over the role of Ellie for a year after Barbara Bel Geddes' heart attack. Bel Geddes returned in the next season.
Jenna Wade was played by three different actresses; Morgan Fairchild for her first appearance in a 1978 episode, Francine Tacker for two episodes in 1980 and finally Priscilla Presley from 1983 onwards a series regular.
Goodnight Sweetheart is a British sitcom about a man who practices bigamy by time-travelling back to the 1940s and marrying another woman there. Both wives' actresses were changed at the start of the fourth season.
Seven different actresses were cast in the roles of Kate's three daughters on Petticoat Junction, with the first cast change happening even before the series started filming (an early example of Sharon Tate's amazing bad luck).
Gary, owner of the rival bar on Cheers, was replaced by a different actor for his second appearance. The interesting twist is that for the rest of the series they kept switching the actors (Joel Polis and Robert Desiderio) back and forth. Since Gary was usually only in one episode a season and the 2 actors looked alike, it's doubtful many people noticed.
Terminator: The Sarah Connor Chronicles, which continued on from the first two Terminator films (but not the third and fourth films), recast every character from the films who appeared on the show. Lena Headey replaced Linda Hamilton as the title character; Thomas Dekker took over the role of John Connor. This also applied to guest characters: Jonathan Jackson replaced Michael Biehn as Kyle Reese (who appears in flashbacks and as a blood-loss-induced hallucination), Bruce Davison replaced Earl Boen (who is retired) as Dr. Silberman, Tony Amendola replaced Castulo Guerra as Enrique Salceda, and a photo of Miles Dyson showed actor Phil Morris rather than Joe Morton.
In the pilot episode, Sarah has a nightmare involving a Terminator played by Aaron James Cash. Although it's not clear, given what Sarah is likely to have nightmares about it's possible this was meant to be the same Terminator model played in the films by Arnold Schwarzenegger.
In similar fashion, the TV series spinoff of the The Net starred Brooke Langton as Angela Bennett, replacing Sandra Bullock from the film.
Long running (more than 4000 20-minute episodes so far!) Hungarian soap Barátok közt (Amongst Friends) has pulled it off three times, mostly combined with a short bus trip. All of them were female characters, first a rape victim visited her relatives in the countryside to forget and another actress came back. Huge lampshade was also hung as a character noticed that she became more feminine. Second, a girl who survived the car crash in which her boyfriend died went crazy, so she took a trip to her relatives in Ireland. The third character wasn't put on the bus, since she was quite new in the show, and the new actress looked similar to the old one. This is also a case of Real Life Writes the Plot, since all of the replaced actresses were in their graduation year.
The actors for every character in Australian TV show Round the Twist was replaced at least once, for the most part because of Real Life Writes the Plot: there was a gap of three years between series 1 and 2 and seven years between series 2 and 3, so all the kid characters were recast due to the actors growing too old; also, the original actors for Tony Twist (Richard Moir) and for Nell (Bunney Brooke) weren't rehired for series 3 because of illness (Richard Moir's Parkinson's Disease and Bunney Brooke's terminal cancer).
Amusingly lampshaded in Dream On. The second season starts with Eddie's face wrapped up in gauze in a hospital room. When the gauze is taken off, he appears to have had some Magic Plastic Surgery. However, everybody says that he doesn't look any different. Martin quips "A little younger maybe."
Granada produced two seasons of The Adventures of Sherlock Holmes with David Burke in the role of Dr. Watson. "The Final Problem" was his last episode, as he wanted to tone down his acting career and spend more time with his family. Edward Hardwicke picked up the role almost seamlessly in the next series, The Return of Sherlock Holmes.
Brian in Seacht was replaced with a completely different-looking actor between the first and second series. His personality also changed.
For one Season 1 episode, Lestat - the Pomeranian dog who portrayed Claire's dog Mr. Muggles - was unavailable, so his brother Beowulf filled in. The two dogs look noticeably different so the new dog was put in a jacket to hide this.
The Burns And Allen Show: Apparently the earliest example of this trope. Four actors had played Harry Morton by the end of the first episode of the fourth season. When Fred Clark, the third Harry Morton, decided to leave the show, George Burns replaced him in the middle of the first episode of the fourth season. In a scene where Blanche Morton was ready to hit her husband in the head with a telephone directory as soon as he came in, George entered, yelled "Stop!" and everyone froze, with the phone book uplifted. George then brought out the new actor, Larry Keating. There was some chitchat, with Clark and Keating saying how much they'd admired each other's work. When George called for the scene to resume, Keating entered and promptly got smacked on the head with the phone book.
In the popular Australian soap Home and Away, the character of Pippa was originally played by curly-haired blonde Vanessa Downing. She was replaced by Debra Lawrence - an actress with straight, brown hair.
In The Restless Years, another Aussie soap, one character was seen in a hairdresser's, commenting "I hope my hair turns out ok". She then whipped the towel off to show that she'd changed from brunette to blonde - and changed actresses at the same time.
The most famous instance concerns Jan in The Brady Bunch Hour. When Eve Plumb was unavailable for the variety series that aired in 1977, actress Geri Reischl stepped into the role.
Parodied in The Simpsons Season 8 episode "The Simpsons' Spin-Off Showcase", said segment being titled The Smile-Time Variety Hour (one of three fictional plots seen in the episode). A blond-haired teenaged girl named "Lisa" replaces the real Lisa, with the explanation that she refused to participate, "But thanks to some creative casting, you won't even notice!"
The other two Brady girls, Cindy and Marcia, have had Other Darrins. In A Very Brady Christmas, Cindy was played by Jennifer Runyon (Susan Olsen had just gotten married in the summer of 1988, when the movie was being filmed, and thus unavailable); in 1990's The Bradys, Leah Ayres took over the role of Marcia (Maureen McCormick declined to reprise her role).
In ER, Dr. Greene's daughter Rachel was played by Yvonne Zima for the first six seasons, then by Hallee Hirsh for all the character's appearances until the show ended. Similarly, Roger, the husband of Dr. Benton's baby mama Carla was played by Victor Williams in a few episodes, before being replaced by Vondie-Curtis Hall. Both were lampshaded somewhat by the fact that recaps of scenes from past episodes were shown, but with any shots of the old actors replaced with the new ones.
As well, when Austin St. John, Walter Jones and Thuy Trang were fired partway through Season 2, a lot of the lines for Red Ranger Jason was done by a decisively different actor mimicking his deep voice during Ranger fights.
In Turbo: A Power Rangers Movie, Divatox was played by Hilary Shepard Turner, who was replaced by Carol Hoyt in Power Rangers Turbo, though Shepard did take over for the second half of Turbo and for Power Rangers in Space.
Reportedly, Miller did show up for the episode, but walked off the set when she found out the Lost Galaxy cast would essentially be playing second fiddle to a child actor. Danny Slavin (who played Leo, the Red Galaxy Ranger) separately walked off in a pay dispute before ADR could be done, so morphed scenes and some other parts of the episode feature the voice actor Christopher Glenn doing his best Leo impression. He later buried the hatchet and appeared in Forever Red in 2002 and traveled to New Zealand in 2013 for a cameo in Series/Power Rangers Super Megaforce.
Lee herself replaced Catherine Battistone between Turbo and In Space.
In Power Rangers Mystic Force, Barbara Goodson was replaced as the longtime voice of Rita Repulsa by Susan Brady, a New Zealand voice actress.
This was Rita's second actress swap, as Carla Perez (who was Rita's body in American footage from season 2 of MMPR to PRIS) took over from Japanese actress Machiko Soga (season 1 of MMPR, via footage from the Japanese series Kyoryu Sentai Zyuranger and some original footage later shot for MMPR). Soga played Rita again in Mystic Force, though, again via Japanese footage.
Decade had the plot conceit of the main characters traveling through Alternate Universes and meeting alternate versions of past Kamen Rider characters, even when the character has the exact same name and backstory (see: Amazon, Tackle in the second movie). It is in fact very unusual for a Decade "Alternate Rider World" character to have the same actor as the original. Exceptions include most the Den-O cast, the original Kiva, and Black/RX.
Over in Super Sentai land, Toei apparently tries not to Other Darrin them unless really needed. A good showing of this would be the Gokaiger Goseiger Super Sentai 199 Hero Great Battle, due to the fact that they were employing nearly 200 stunt actors for the Great Legend War scene, they had the Gokaiger and Goseiger actors cover the non-guest starring Sentai heroes.
Before actor Zachary Quinto was cast as Sylar, the role was being played by various stunt doubles (usually noticeably older than Quinto) being shot from behind, half-obscured, and with a baseball hat. A voice message left by Sylar before Quinto was cast was recorded by voice actor Maurice Lamarche.
Similarly, actor Erick Avari had not been cast as Chandra Suresh when the first few episodes were filmed. Whenever Chandra appears in photographs (with Mohinder or on the back cover of his book "Activating Evolution"), different actors are used; one bearded, one in a turban. Some of these were later replaced by photos of Erick Avari for the DVD releases, but some were missed.
Twitch City recast the role of Rex Reilly between Seasons 1 and 2, lampshading it when talk show host Rex mentions his "on-air cranium transplant." However, this explanation is undone by the season premiere, for which a flashback to Nathan's appearance on the talk show was reshot with the new Rex.
On The New Adventures Of Robin Hood, Barbara Griffin replaced Anna Galvin as Marion in Season 2, and then John Bradley replaced Matthew Poretta as Robin in Season 3.
1990's soap Sunset Beach have a bunch of these. One of the main characters got switched twice.
Bones had this with just a bit part. Benito Martinez (Capt. Aceveda of The Shield) played a one-off chacter Vega, an author whose fame came from following, and making famous, a serial killer "The Gravedigger". This killer was never caught, and abandoned for a few seasons. Suddenly "The Gravedigger's" back, and all suspects are rounded up again. Included is Vega, now played by Marco Sanchez. He's there mainly to be killed off halfway through, but they hardly looked alike beyond being tall and Latino.
Michelle, Cam's adopted daughter is portrayed by Dana Davis, who got a starring role on another show for the next season and was replaced with Tiffany Hines.
The Saturday Night Live sketch The Whipmaster, while not a real example as there was only one Whipmaster sketch, is about how this trope can go horribly, horribly awry.
On the actual show, Fred Armisen's Barack Obama impression has recently been replaced by Jay Pharoah's take on him.
Invoked on The Famous Jett Jackson when Executive Meddling wants to bring more gun-based violence to the Show Within a Show "Silverstone", which the actor playing the villain of the specific episode debuting the gunplay disagrees with the change. He is obviously fired and a new actor is brought in, the new actor's debut scene includes Jett's character Silverstone commenting on the villain's plastic surgery. Of course, the episode has An Aesop of "guns are bad" (Jett's father, a sheriff, had to shoot a criminal on duty and is traumatized by the event), so in the end Jett and the entire remainder of the crew threaten to quit, so the producer has no choice but to scrap the episode and ditch the guns idea. The original actor returns to film the original script of the episode, thanking Jett for his support.
LOST may or may not have invoked this trope. When Ben and Locke visit Jacob's cabin in Season 3, Jacob, in his split-second appearance, is "played" by the show's prop master Rob Kyker, who has long hair, a beard, and appears to be in his 40s or 50s. He looks absolutely nothing like Mark Pellegrino, the actor who plays Jacob in his many appearances in the Season 5 finale. However, there is evidence to suggest the man in the cabin was not Jacob. Additionally, Titus Welliver, who plays Jacob's unnamed nemesis, does bear a resemblance to Kyker, and is possibly the individual who impersonated Jacob in the cabin. If that is true, this would still be a case of The Other Darrin.
Done with Charlie's dad: made acceptable because you don't see his face the first episode.
Same case with Claire's mother: she has no lines in her first appearance and is only visible under bandages.
Dr. Quinn, Medicine Woman had an incident midway through the show's run where Erika Flores, who played Colleen, was abruptly fired and replaced by Jessica Bowman. Needless to say, the fandom was furious.
Being Human has "The Other Darrins". Two of the main trio and one other important character were replaced between the pilot and the series. Aidan Turner replaced Guy Flanagan as Mitchell, Lenora Crichlow replaced Andrea Riseborough as Annie and Jason Watkins replaced Adrian Lester as Herrick. These last two are especially noticeable because Lenora has tan skin and dark hair, while Andrea is pale and blond, and Lester is black while Watkins is white. The curious thing is that the pilot is still part of the plot, but once the series proper started they didn't re-shoot the episode so much as make new scenes with the new characters to bridge the gap.
The Inspector Lynley Mysteries had Lesley Vickerage as Helen, up until she separated from Tommy. Then in the last season she comes back - played by Catherine Russell. This wouldn't be so bad if the two actresses looked anything like each other - one notable difference is that Lesley's hair is short and black while Catherine's is long and brown - and if they hadn't called attention to Helen's appearance in an episode. When you make Havers get haircut to look like Helen as a cover, and you make a big deal about how they look the same from behind, chances are fans will notice. Better yet, Vickerage was actually the second actress to play Helen — the character also appeared in the first story ever shot, the pilot episode "A Great Deliverance", played by Emma Fielding!
In the pilot for WKRP in Cincinnati, the mother of the station manager was played by Sylvia Sidney. She wasn't available for the series because of other acting commitments, so she was replaced by Carol Bruce, who was about a decade younger and much taller than Sidney. A subsequent Clip Show then had the scene from the original pilot re-shot with Bruce.
The roles of Quinn's parents on Sliders changed hands almost every season.
Brain-tissue-harvesting Evil Brit Col. Rickman was played by Roger Daltrey in his first appearance. Perhaps realising it might have been wiser to cast an actual actor, Neil Dickson replaced Daltrey as Rickman in all his subsequent episodes.
Brass's daughter Ellie in CSI: Crime Scene Investigation was played by two very different-looking actresses — Nicki Aycox◊ in her first appearance, Teal Redmann◊ in her subsequent ones. The fact that there was a multi-season gap between her appearances took the edge off of this somewhat.
CSI NY had a variation on this. The woman whose photo was used as that of Claire, shown to Reed by Mac in season 4 is not the actress who played Claire in the season 8 eps 'Indelible' with its flashbacks and 'Near Death' with Mac's Near Death Experience encounter with her.
Lucy Messer borderlines,the need for a different actress as she grew older was natural,but even when there was only 6 months skip between s8 and s9, a new actress was still used.
In Dynasty, all four of Blake's children switch actors.
On the old Batman live-action TV series, the role of Catwoman went from Julie Newmar to Lee Meriwether for Batman: The Movie, and then in the final season it went to Eartha Kitt.
In his three appearences, Mr. Freeze was played by George Sanders, Otto Preminger, and Eli Wallach.
John Astin replaced Frank Gorshin as The Riddler in one story.
The Australian fake lifestyle show Life Support used this a lot.
Penne was played by Abbie Cornish during season one, but replaced by Alison Barnes for the second and third seasons. This was heavily lampshaded when Todd suspected that "something was wrong" with Penne, but Sigourney reassured him by showing flashbacks from the first season.
When the character of Trapper John McIntyre (played in the movie by Elliott Gould) left Mash, he reappeared in the stateside medical drama Trapper John MD. Wayne Rogers played him on M*A*S*H; Pernell Roberts on Trapper John. (Justified in that the latter series takes place some 25 years after the former and thus involved a much older character.)
Speaking of M*A*S*H: Father Mulcahy was played by George Morgan in the show's pilot before William Christopher took the role.
Not to mention that only Radar and a few minor characters are played by the same actor in the film and the TV series. Every other major character is an example of the trope.
Margaret Houlihan's fiancé, Lt. Donald Penobscot, was played by a different actor in each of his two appearances on the show. Ditto visiting nurse Meg Craddy.
Three different actresses played Rosie (of Rosie's Bar) during the show's run.
There were lots of different actresses who played "Nurse Able" or "Nurse Baker" in various episodes.
Mildred Potter (Colonel Potter's wife) appeared in M*A*S*H only as a photograph (of Harry Morgan's real-life wife Eileen Detchon), but in the unsuccessful spinoff AfterMASH, she was played by Barbara Townsend in the first season and Anne Pitoniak in the second (the OTHER Other Darrin?).
In the Australian serial drama Ocean Girl the mother of two of the central characters is recast after Season 2 to be played by a different actress in Seasons 3-4 without explanation.
In FlashForward (2009), the actor playing Dylan Simcoe was recast in the middle of filming the pilot. So some scenes feature the first and some the other, and the credits credit "Dylan #1" and "Dylan #2". The second one has been consistent ever since.
In The Waltons, Robert Wightman took over for Richard Thomas as John-Boy in the last two seasons of the show. Thomas eventually returned for a series of early '90s reunion movies, however.
Halfway through Filmation's live-action series Shazam!, Jackson Bostwick as Captain Marvel was fired and replaced with John Davey.
Deirdre Barlow's daughter Tracy in Coronation Street was portrayed by child actresses Christabel Finch and Holly Chamarette before switching to Dawn Acton from her late teens onwards. All three portrayed the character in a broadly consistent way as moody, insecure and eventually evolving into something of a Goth. From 2002, she was played by Kate Ford.
Tracy's stepsiblings Peter and Susan Barlow (originally played by actual twin babies) have been repeatedly Other Darrined since they were sent to live with their late mother's parents at the age of six in 1971. At one point, the original actress (apart from the baby) to play Susan returned to the role, at another, Peter was played by the real life son of Ken Barlow's actor, who has since taken the role of another of Ken's sons.
Ned Banks on Ghost Whisperer. Introduced in Season 2 with actor Tyler Patrick Jones in the role, who was replaced by Cristoph Sanders in Season 3. Noteworthy because Sanders looks nothing like Jones, being taller, blond (Jones has dark hair), and noticeably older.
Has occurred with the characters of Adam (Jon Fleming, replacing Stephen Amell) and Michelle (Jill Bennett, replacing Erin Cummings) in the here! network's softcore horror soap Dante's Cove. In both cases the recasting extended to having the new actors re-film key scenes from the previous season for use in the show's opening recap.
Anita Briem played Jane Seymour during the second season of The Tudors. In Season 3, Jane Seymour is played by Annabelle Wallis.
A Nero Wolfe Mystery's pilot episode, "The Golden Spiders", has Saul Rubinek playing Wolfe's freelance associate Saul Panzer, and Gerry Quigley was playing the reporter Lon Cohen. The first episode after it becomes a series, The Doorbell Rang, has Rubinek playing Cohen, and Conrad Dunn plays Saul Panzer. This casting remains consistent for the rest of the series.
In May 2010, Erin Mullally replaced James Sorensen as Declan Napier.
In 2008, the actress playing Libby Kennedy (Kym Valentine) contracted severe pneumonia and was replaced for four weeks by Michala Banas, with no explanation given to the audience.
Additionally, for people who grew up watching Neighbours, the relationship between Scott and Charlene and the real-life romance between their actors, Jason Donovan and Kylie Minogue, was the main event of the late eighties ... but Jason Donovan wasn't the original Scott. That was Darius Perkins.
Todd Macdonald replacing Scott Major as Darren Stark.
Wonderfalls does this with Eric's wife Heidi. In Eric's honeymoon flashbacks she's played by Corry Carpf. Later, when the character actually appears in the series, she's played by Jewel Staite.
The original version of Survivors had at least three characters replaced; two of the switches (Ruth and Lizzie) happened between seasons, while Vic had to be replaced when his first actor had a nervous breakdown.
The youngest daughter of Bill and Barb Henrickson in Big Love suddenly grew up and changed appeareance in the fourth season; Jolean Wejbe was replaced by Bella Thorne. No one mentioned anything.
Farscape spoofs this in "John Quixote" by having an overweight male actor playing the role of Zhaan (as a piece of misdirection for the original actress appearing later in the episode).
The Peacekeeper Wars miniseries introduces a different actor to play Jothee, who had last appeared two seasons previously. Slightly lampshaded by John Crichton remarking how much Jothee had grown.
Veronica Mars originally aired with Backup, the title character's dog, being portrayed as a bulldog (by a dog called Gordo). Subsequently it was a pit bull (played by a dog called 'Lefty') - not only a different colouring entirely (changing from brown spots on white to tan all over) but a breed change. This was because, despite having one and a half months to prepare, the trainer had failed to teach the original dog-actor the required stunt and so the dog had to be replaced. However, no hint was given in the show that anything was amiss, despite the replacement dog also being much larger than the original.
The 1960s sitcom The Mothers-in-Law replaced without comment one of the principal actors, rotund, mustachioed Roger C. Carmel (Harry Mudd on Star Trek) with bald, bespectacled Richard Deacon (Mel Cooley on The Dick Van Dyke Show).
Yet oddly subverted with Vorena the Elder, who is played by the same actress through the entire series, despite the fact that her character starts out younger than Octavian and should by rights be at least in her mid-thirties by the time the series ends. (They do play it somewhat straight with young Lucius and Vorena the Younger, casting older children, but neither is even close to the age they should be by the end of the series.)
All My Children, like all daytime soaps, has recast characters countless times. However, in 2007 they decided to have fun with it by transitioning actresses playing Babe in mid-hug with her mother Krystal. Alexa Havins went in for the hug and Amanda Baker pulled back from it. See it here.
It is hilariously parodied a couple of episodes later when both the actors show up at the same time as Wagbo to terrorise Harry.
And the original Wagbo does so with an obvious sunburn on his face.
On Back To You, Lily Jackson replaced Laura Marano as Gracie a little over halfway through the season.
Played straight and lampshaded with Allison Stark in Til Death, even going as far as (sort-of) namedropping the trope and occasions on other shows where this has happened. Allison's boyfriend has a minor case of Medium Awareness and is therefore the only character on the show to notice the change.
Somewhat lampshaded in Meet The Browns when Logan Browning replaced Brianne Gould as the character Brianna for the second season. Upon first seeing her, Brown asks, "Who are you?" Brianna replies, "I'm Brianna." To which Brown says, "You a lie!" He seemed to be the only one to realize that the actress for the character had been switched.
Spartacus: Blood and Sand is a strange example, in that the changeover was publicly announced by the actor and the showrunner well in advance of the new actor's appearance: Liam McIntyre is replacing Andy Whitfield in the titular role of Spartacus for Season 2 after the latter was diagnosed with non-Hodgkin lymphoma (which proved to be fatal in September 2011), and therefore couldn't return for the production of the show's second season.
It could be because the show is made in New Zealand, or that the show's execs really didn't want to recast for the role of Spartacus since Whitfield had done such a stellar job in the role. The show even went on to do a prequel (Spartacus: Gods of the Arena) while Whitfield got treatment for his cancer. Unfortunately it didn't go into remission, so he bowed out and even recommended McIntyre to replace him.
Lesley-Ann Brandt, who played Naevia, was replaced by Cynthia Addai-Robinson in the same season, apparently due to scheduling conflicts. While Spartacus being replaced was justified due to him being the main character and all, the showrunners could have easily avoided recasting Naevia. Her Season 1 storyline ended with her being essentially Put on a Bus, and the writers could have simply stalled Naevia's rescue until Brandt was able to return, avoiding a Replacement Scrappy in the process.
Days of Our Lives did an unusual take on this. Philip Kiriakis was played by Jay Kenneth Johnson until 2003 when he was replaced by Kyle Brandt in the usual manner. In 2006, Phillip had been injured in the military and had to receive facial surgery that made Him look like a different person and was then played by... Jay Kenneth Johnson.
On the Emmy-winning public access sitcom Elvis And Slick Monty, the actor for Elvis changed after the pilot, then again after the first season concluded. This was lampshaded when the second season opened with the characters watching the 10th season premiere of Family Matters. Slick explained the reason for the new Harriet, but Elvis would have none of it.
In the pilot for Blossom the title character's dad is played by Richard Masur, who doesn't look much like Ted Wass (who played the dad in the series).
Network executives didn't like the idea of Dirk Benedict playing Faceman in The A-Team; which is why Tim Dunigan plays him in the pilot.
The governor of Hawaii in the pilot for the original Hawaii Five-O was played by Lew Ayres; for the series he turned into Richard Denning - but oddly enough, Lew Ayres later appeared on the series in another role!
Staying with Five-O, Tim O'Kelly played Danny Williams in the pilot but was replaced with James MacArthur for the series; and the recurring character of State Department official Jonathan Kaye was played by six different actors (including the very-dissimilar types of James Gregory and Tim O'Connor).
In the gay vampire series The Lair the actor for the character Jonathan changed between Season 1 and Season 2 and the actors looked so unlike (they had the same hair colour) that it was impossible to tell they were even the same person until the second was called Jonathan. The second actor was only used for one episode due to the character being written out.
Mrs. Peacock, who makes appearances in a few Are You Being Served? episodes, was played by two actresses.
The Adventures of Robin Hood had two different actresses play the role of Maid Marian: Bernadette O'Farrell in series 1-2, and Patricia Driscoll in series 3-4.
In the soap Another World, the role of Rachel Davis Matthews Cory was originally played from 1967 to 1972 by Robin Strasser (who later played Dorian Cramer Lord on One Lifeto Live). Victoria Wyndham took over the role in 1972, and she became the best-known Rachel Cory.
On Home Improvement, Jill's sister Robin appeared in two episodes and was played by a different actress each time.
In Carrusel, when Fermin's first actor (Augusto Benedico) was no longer available, he was replaced by Armando Calvo- who was and looked 10 years younger than the first actor. Also, the actress who played Maria Joaquina's mother Clara was replaced.
On Grace Under Fire, the title character's son Quentin was played by three different actors - Noah Segan in the pilot, then Jon Paul Steuer, and Sam Horrigan in the final two seasons. The second switch is notable for also being a case of Soap Opera Rapid Aging Syndrome, as Quentin went from a pre-teen to a fifteen year old.
When Gleason was on the Du Mont network's Cavalcade of Stars, Pert Kelton played Alice. Kelton's husband had been blacklisted when Gleason moved to CBS, and her career suffered due to guilt by association.
Audrey Meadows, who is most identified with the role, took over when the show debuted on CBS.
Sheila MacRae assumed the role in a couple of TV specials between 1966 and 1973.
Elaine Stritch played the role in the first outing on the DuMont show.
Stritch was replaced by Joyce Randolf, who is most identified with the role.
In the 1966-73 specials, Jane Kean assumed the role.
In the first season of the original series, Jeannie's mother was played by, in "My Hero", Florence Sundstrom; in "What House Across the Street?", Lurene Tuttle. In the fourth season, expanding on the trope of Barbara Eden herself playing Jeannie's sister, she twice played her mother.
Gilmore Girls was pretty good about keeping actors that played recurring characters (and even recycled actors when the character would leave the show) except in two cases, one notable and one you could be forgiven for not noticing.
Mia, an important background character, who was the owner of the Independence Inn and a surrogate mother for Lorelai and Rory appeared in Season 2 for a single episode by Elizabeth Franz. When Mia appeared in the final season, she was suddenly played by Kathy Baker with no explanation.
Cesar, one of the servers at Luke's Diner, would occasionally appear in the background for the first two seasons played by a tall, thin actor, but had no scenes or lines. In the later seasons, he was played by Aris Alvarado, who was chubby and looked nothing like the first actor.
Disney's The Swamp Fox series replaced the actress who played Marion's fiance, Mary Videaux, after two episodes. The first was a light blonde, and the second had auburn hair.
While many members of the cast of Clueless appeared on the series either in their film roles (Elisa Donovan, Stacey Dash, Wallace Shawn) or guesting as other characters (Paul Rudd, Breckin Meyer, Brittany Murphy), for the ABC episodes Dan Hedaya (Cher's father) was replaced by Michael Lerner; when the series moved to UPN he in turn was replaced by the more handsome and much slimmer Doug Sheehan (his character was said to have gone through detox!). Alicia Silverstone, on the other hand, was replaced with Rachel Blanchard throughout.
Happened with the voice over included in One Life to Live, when the original Todd Manning was replaced by another actor.
The trope was turned on its head when James De Paiva, the original Max Holden, was replaced by Nicholas Walker. The show explained that he looked different because of an accident which had required plastic surgery to replace Max's face. But then De Paiva came back to the show, and there was no explanation as to why he looked like his old self.
Bob West, the familiar original voice of Barney, retired from the role in 1999, then Duncan Brannan took over for three years, then he was replaced by Dean Wendt.
The One, the Big Bad in the second season of Martial Law, was initially never seen and only heard speaking with the voice of Tim Curry. When he eventually showed up to take care of business personally not only did he look like fellow British actor Christopher Neame (because a climactic fight between Sammo Law and Tim Curry would have too ridiculous for words, thanks to Curry not exactly being the physical type), but the "Previously on Martial Law" bits featuring Curry's voice were redubbed by Neame (and it's very noticeable).
Midway through Tinsel Season 2, Uzo Egereonu left the show and Funmi Holder took over the role of Amaka Okoh.
Funlola Aofiyebi-Raimi replaced Juliet Asante as Brenda Mensah.
In Season 5, Leonora Okine replaced Mathilda Obaseki in Season 5 as Angela. Also an example of Replacement Scrappy.
During a dispute with the producers, Clayton Moore was replaced with John Hart as The Lone Ranger for one year. Moore eventually returned to star for the rest of the series' run.
Buffy the Vampire Slayer replaced the actress who played Nikki Wood in season 5's "Fool for Love" with another actress for her several appearances in season 7.
There's also the pilot where Willow was played by Riff Reagan. Alyson Hannigan took over in the main series.
And Buffy's first watcher,Merrick. the actor playing him in Buffy and Angel's flashbacks was very different from the movie's Donald Sutherland. Probably because Whedon imagined him like the series version and the filmmakers changed it.
Game of Thrones lost the services of the 7' tall Conan Stevens in the role of Gregor Clegane after the first season, as Conan was going to New Zealand to do stunts for The Hobbit. Luckily, the 7'1" Ian Whyte was known to the producers from his earlier appearance as a White Walker and stepped into the part. Whyte may have been the same height, but he was noticeably more slight (the character's nickname is "the Mountain that Rides"), but on the plus side, he had a much deeper voice. For season 4 the part was recast again, this time with Iceland's Strongest Man Hafţor Bjornsson (6'9" for those keeping score at home).
Also in season four, Michael Huisman will be taking over the role of Daario Naharis from Ed Skrein, whose multimedia career is likely to have left him unavailable for the role, and Dean-Charles Chapman will be taking over the role of Tommen Baratheon from Callum Wharry, making for a rare case of an actor being both The Other Darrin and You Look Familiar (as Chapman previously portrayed Martyn Lannister, a character with a comparatively small part).
Cadfael had a different actor play Hugh Beringar in each of its three seasons: Sean Pertwee, Eoin McCarthy, and Anthony Green. Derek Jacobi says he naturally has a favorite of the three but won't say who.
The television adaptation of The Aldrich Family, which ran from 1949 to 1953, had so many cast changes that each season featured an almost entirely new central cast. In all, fourteen different actors and actresses were rotated in and out of four of the five major roles:
Protagonist Henry Aldrich was played by Robert Casey, Richard Tyler, Henry Girard, Kenneth Nelson, and finally Bobby Ellis.
Henry's sister Mary was played by Charita Bauer, Mary Malone, and June Dayton.
Henry's mother Alice was played by Lois Wilson, Nancy Carroll, and Barbara Robbins.
Henry's best friend, Homer Brown, was played by Jackie Kelk, Robert Barry, and Jackie Grimes.
The lone aversion was Henry's father Sam, who was played throughout the series by House Jameson (who also played the role on radio).
Glee's Season 3 launch released a slew of fandom-bending changes, none explained with so much as a hand wave, and the more jarring of which was Rachel Berry's dads - featured in a series of photobooth pictures in the series pilot by two Hey It's That Guys! - would be played mid-season by Jeff Goldblum (whose pilot doppelganger was bald) and Brian Stokes Mitchell (whose pilot doppelganger was of a significantly darker complexion).
The TV series Assignment: Vienna had its entire regular cast made up of Other Darrins - the pilot film, Assignment: Munich (even the location was a case of The Other Darrin) starred Roy Scheider, Richard Basehart and Werner Klemperer, but Scheider elected not to do the series because he thought films were his future (Sea Quest DSV notwithstanding, he was right); hence all three leads were replaced by Robert Conrad, Charles Cioffi and Anton Diffring respectively, with filming changed to Vienna so the 1972 Olympics wouldn't disrupt production.
Phyliis (one of three spinoffs of The Mary Tyler Moore Show) had one of the most tragic examples of this trope: Barbara Colby (who played Phyllis Lindstrom's boss) was replaced by Liz Torres when she was killed in a drive-by shooting after four episodes had been produced.
Season 2 of Last Man Standing saw some of this, the most notable example being the older daughter Kristin, who's actress changed from Alexandra Krosney to Amanda Fuller for "creative reasons". In addition, her boyfriend Ryan was played by Nick Jonas as a guest star in Season 1, but has since been played by Jordan Masterson after the character became a regular. In addition, their son Boyd was originally played by the twins Evan and Luke Kruntchev for his Season 1 appearances, and by Flynn Morrison after the character was slightly aged up in Season 2.
Ed McMahon was the original host of NBC's Missing Links in 1963. He was replaced by Dick Clark when the show moved to ABC six months later due to his comittments with NBC.
On Concentration, Hugh Downs turned the reins of hosting over to long time announcer Bob Clayton at the start of 1969, but NBC wanted to go with McMahon. When ratings started to tumble, NBC switched to Clayton.
From, strangely enough, The Price Is Right. For the first three seasons, there were two models; Janice Pennington (a blonde) and Anitra Ford (a brunette). Around late July 1975, Anitra took a leave of absence and another model filled in: specifically, a blonde named Dian Parkinson, who had previously filled in for Anitra in mid-April (and hence, a natural choice). Not-so-naturally, beginning on August 19 she started to wear a dark wig and be addressed as "Anitra". Even with a wig there wasn't really much resemblance, so why Price felt they needed to do this is a mystery. When Anitra came back between early September and late October, Dian became the show's third model and was allowed to be herself again.
Original Wheel of Fortune host Chuck Woolery left in 1981 after his request for a raise was turned down. Pat Sajak, then meteorologist for Nashville TN's NBC affiliate WSMV, has been the host of the syndicated nighttime show since. He hosted the daytime show up until 1989 when he departed for his ill-fated CBS late night show. San Diego Charger placekicker Rolf Benirschke filled the host spot until July 1989 when the show switched to CBS and Bob Goen took over. Goen remained with the daytime show through its return to NBC and its course run to September 1991. On the same show, original hostess Susan Stafford quit in 1982, and Vanna White (who still holds the role to this day) took over.
Garry Moore pulled this twice. In 1966, he left I've Got A Secret and turned the moderator seat over to Steve Allen. In 1977, he retired from TV and after hosting the syndicated To Tell the Truth since 1969 was replaced with Joe Garagiola.
The NBC 1990 reboot of To Tell the Truth saw the host chair filled thrice, first with Gordon Elliot, then Lynn Swann, then for the balance of the show with Alex Trebek. On top of that, the pilot (hosted by Richard Kline) accidentally aired on the East Coast, and Mark Goodson had to fill in for Trebek on two episodes because Trebek's wife had gone into labor.
Wally Bruner left as host of the syndicated What's My Line? in 1974 and was replaced with Larry Blyden for the show's final season. Speaking of Blyden, he was to host ABC's Showoffs that summer of 1975 but was killed in an auto accident. The clips used in ABC's promos for the show had Blyden hosting but was he was never shown in those clips. The host job went to Bobby Van.
The Name's The Same (ABC, 1951-55) went through Robert Q. Lewis, Clifton Fadiman, Dennis James and radio comedians Bob Elliot and Ray Goulding as moderators.
The Timmy era of Lassie started with Cloris Leachman (of The Mary Tyler Moore Show fame) and John Shepodd as Timmy's parents and later replaced them with the more familiar June Lockhart and Hugh Reilly about a season in.
On The King of Queens, Deacon and Kelly's sons were played by several different actors over the course of the series. Then again, they were extremely young, so most casual viewers probably didn't notice.
Also happened with Spence's mom Veronica. In her first appearance, she's played by actress Grace Zabriskie, but in all her subsequent appearances she's played by Anne Meara (meant to be an in-joke, since Meara is Jerry Stiller's Real Life wife, and Veronica was depicted as being in love with Stiller's character Arthur). What makes it really Hilarious in Hindsight is that in that first episode where Zabriskie had played Veronica, Meara had appeared as a completely different character who had also harbored a crush on Arthur!
How I Met Your Mother: Robin's father was portrayed by Eric Braeden in season 4, but is currently played by Ray Wise. Tom, Barney's brother-in-law, was played by an unnamed actor for a picture◊. Later on season 7, he was portrayed by Jai Rodriguez, creating a weird ethnicity◊ switch.
Drake & Josh: Helen Dubois is usually played by Yvette Nicole Brown, but once was played by Frances Callier in the episode Little Diva, from the second season.
Nineties British sitcom Birds of a Feather about two sisters coping together while their husbands were in prison had two cast changes, in that both actors who played the husbands were replaced. Chris was recast for Series Two up until the final series, at which point, Daryl was replaced as well while Chris was re-Darrined back into his Series One actor.
The pilot for the original Law & Order, shot in 1988, cast Roy Thinnes as Manhattan DA Alfred Wentworth. When the show made it to series two years later, Thinnes was unavailable, so Steven Hill was cast instead as DA Adam Schiff. While technically, the two are different characters and the trope wouldn't normally apply, NBC rather inexplicably aired the pilot in the middle of the show's first season. Thus, when viewing the episodes in their original broadcast order, the DA suddenly changes to a completely different person and back again with zero explanation.
In one episode of Law & Order, ADA Claire Kincaid was portrayed by Jacqueline Hennessy, the identical twin of Jill Hennessy, who usually played the character. Somewhat unusual for this trope in that it's not noticeable unless one is actively looking for it.
The pilot episode of Walker, Texas Ranger cast Gailard Sartain as C.D. Parker. In episode 4, Noble Willingham replaces him and continues the role until he would pursue politics, leading to the Put on a Bus / Bus Crash of his character.
This trope is so rare in Latin America television, it was somewhat of a minor scandal when the Mexican Soap OperaNada Personal changed the actress of the main heroine,from Ana Colchero to Christianne Gout, with only an small explanation at the beginning of a chapter who bordered in a Lampshade Hanging.