Maia on The 4400. She was one of the most prominent 4400's when the show began, but eventually her role was reduced to being Diana Skouris's daughter and morality pet, along with the occasional prophecy. This is likely due to a couple of reasons. One of which is the mysterious nature of the show, and as Maia's abilities became more powerful she would essentially be able to solve most of the shows big mysteries in minutes instead of over the course of several episodes. The second reason is child labor laws...
Deal or No Deal: The 25 models were the main attraction of the game show, but when the show entered syndication and the prize money was reduced, all but two of the models were replaced with contestants hoping their number would be picked so they can play the game, and if not, the contestants would open the cases during the game. The two models used in this version were for only spinning the wheel to see who can play and removing cases that were picked.
The Lawrence Welk Show: The legendary bandleader always liked to spotlight many of his new stars of his orchestra during a given show, giving them solos. Legend has it that Welk used this as a probation period for his new stars, and used audience reaction (both in-studio and viewer mail) to determine if the band member was worth keeping. If they were, they were featured more prominently ... and sometimes, even given longer solos. If not, ... well, the unlucky band member – regardless of his actual talent – began fitting the trope before being fired.
On Boy Meets World Alan, Amy, and Morgan Matthews are demoted during seasons 6 and 7 when the main characters went to college.
Mr. Turner appeared less often and had less impact on storylines in the show's fourth season before disappearing completely by the fifth season.
After Katey Segal's tragic miscarriage on Married... with Children, where the producers had written her pregnancy into the storyline before being being forced to backtrack, the character of Peg Bundy was instead subjected to this trope during the second and third times Segal became pregnant during the show's run. Both times, Peggy was sent out in search of her missing father, and was only shown in voiceovers talking to her family on the telephone, or only shown in scenes where she didn't have to do anything strenuous and the viewers couldn't see her pregnancy. Happily, Segal's later two pregnancies were both successful, and when she was ready to return to work the producers brought Peggy home, much to Al's chagrin.
After Jack Harkness left the TARDIS and subsequently joined Torchwood, he made return appearances in the third and fourth series' season finales. Similarly, both Martha and Rose returned for the series 4 finale (along with Martha making prior guest appearances in both Doctor Who and Torchwood) after officially "leaving" their roles of companion.
Interestingly, they're not seen as being treated unfairly though technically qualifying for this trope. In the old days, past companions seemed to not exist anymore, even when they'd traveled with the Doctor for a long time, or taken part in important events. The new series avoids this. After Martha's departure, Powers That Be got much smoother at finding where characters who were no longer with the Doctor (but should still be around when we visit present-day Earth) belonged.
Roger Delgado's incarnation of the Master. In his first season of Doctor Who, the character turned up in every single serial, from Terror of the Autons to The Daemons. Then, Delgado, while enjoying the show, became concerned that while officially a guest star, many casting directors considered him a de facto regular cast member of Doctor Who and therefore unavailable for other work. So in the next season dramatically scaled back his appearances, with an eye to making a splashy departure the following season. Due to his untimely death in Turkey, the character was quietly retired for a time.
After Don Knotts' departure from The Andy Griffith Show as a regular, his character of Barney Fife was brought back as a guest for at least one episode in each of the remaining seasons.
Though still listed as a main cast member, Desmond was commuting on a bus to make brief appearances in a few episodes in the fifth season. Still listed as a main cast member (on the episodes, not in the press releases) in season six... he showed up in the first episode, but didn't show up again until the second half of the season.
Frank Lapidus was also Demoted To Extra. He was a pretty major character during Season 4 and most of 5, but when Season 6 came around he seemed to have gotten lost in the shuffle and became an extra body and frequent cheesy One-Liner. Pretty ironic considering Jeff Fahey wasn't promoted to the main cast until Season 6. Made worse by the fact that we never really knew who Lapidus really was or where he came from, as he was one of the few main characters to never get his own flashback episode. The one flashback he did get was in an episode that he shared with several others, and all it did was explain how he got onto the freighter that came to the island.
Steven Hill was originally the main star of Mission: Impossible, but partway through the first season (after, among other things, his returning to his roots of Orthodox Judaism meant that he refused to work after sundown on Fridays) he was downgraded to only appearing in a couple scenes per episode. And then he became the first - but not the last - member of the IM Force to fall victim to Chuck Cunningham Syndrome.
During the final season of The Bob Newhart Show, there are several episodes where Newhart himself has only one scene, which is not shared with any other characters (allowing him to perform one of his trademark one-sided telephone conversations).
Very prevalent in the weird final season of Welcome Back, Kotter. Mr. Kotter only makes fleeting appearances, due to actor Gabe Kaplan not being very happy with the show. No one acted as though it were at all unusual that Mr. Kotter hardly seemed to be around any more. In fact, Gabe Kaplan still got top billing. At least Barbarino, who was also absent that season because John Travolta was doing movies, was explained away as having dropped out of school. To preserve some semblance of familiarity, the previously stay-at-home Mrs. Kotter became the Sweathogs' new adult supervisor in her role as the school secretary, or something.
This started happening to Kimberly in season three of Mighty Morphin' Power Rangers. Notably, she was the only one who didn't go to Edenoi during the season premiere (AKA the Poorly Disguised Pilot for Saban's Masked Rider) due to a cold (though she made use of reduced screentime awesomely, being the only Ranger left on Earth when Rita and Zedd sent a new Monster of the Week), and had her powers drained halfway through after being captured by Kat Hillard. Since Amy Jo Johnson actually told Saban she was going to quit beforehand, instead of clumsy camera tricks and an abrupt Fake Shemp, half the season was spent slowly easing her character out of the Pink Ranger position and setting up her successor.
After Skull was Put on a Bus in Power Rangers Lost Galaxy, the writers apparently had trouble coming up with material for Bulk on his own. He only appears in a few episodes despite being in the opening credits. He would not have a recurring role again until Power Rangers Samurai 12 years later.
In Power Rangers S.P.D., Sam the Omega Ranger was demoted compared to his counterpart DekaBreak from the source material. Disney forced a Fake Shemp situation that the writers didn't want to deal with, and they wouldn't have included Omega Ranger at all if they weren't stuck with so much DekaBreak Stock Footage.
In the seventh season of Red Dwarf, Arnold Rimmer only appeared in person in two episodes, then was relegated to flashbacks, dream sequences and—in one memorable case—a theme park ride based on his own diaries. The actor playing him, Chris Barrie, had decided to leave the show and had a very limited schedule, so he agreed to appear in a handful of episodes (and sequences that could be filmed out of order, such as ADR or flashbacks).
On House, Doctors Cameron and Chase were fired from the title character's team at the end of Season 3. In Season 4 they were shown to have taken new jobs at the hospital, while House recruited a new team.
Season 6 has a lot of demotions: Cameron becomes a long-distance extra, while Chase found his way back into the cast.
On Angel, Cordelia spent a good portion of her last season in a coma despite nominally being in the opening cast.
Speaking of Angel, his last season on Buffy the Vampire Slayer had very reduced screen time (though being in hell is quite a commute).
This also happened to Giles in the last two seasons of Buffy.
The Secret Circle. Nick was Cassie's love interest in the books. In the series he is quickly killed off and replaced by his older brother Jake (who wasn't in the books).
Jane was also prominent in the first nine episodes. Since then, she's only appeared a couple of times before getting killed off like Nick.
30 Rock. Josh Girard, who was prominent in season one as Jenna's male co-star, and reduced to an almost anonymous staff writer.
The 100th episode lampshades this with the new actor Danny having replaced Josh on the show-within-a-show as the "other guy", no explanation for Josh's complete departure is given, and everybody keeps forgetting Danny wasn't the "other guy" that was with them from the beginning (even Danny starts to mix himself and Josh up under the effects of the gas leak).
Josh actually reappears briefly before that, when he throws a fit over how he's been neglected and ignored and quits - then later, auditions for his own old job back and loses to Danny. Danny ends up befalling the same fate anyway.
Richard Dean Anderson's final season as a regular on SG-1 had him promoted to general where he had vastly reduced screen time.
Stargate Continuum opens with an over-the-shoulder shot of Pentagon liaison Major Davis walking down the corridors of Stargate Command. He was a fairly major recurring character in the earlier seasons of the TV series, so you might expect him to have at least one scene, but nope, the camera quickly cuts to the main characters, and all you see of Davis in the movie is the back of his head for several seconds. The director did this as a joke, according to the DVD commentary.
Teal'c is the only member of the original SG-1 who did not appear in Stargate Universe.
Meanwhile, of all the characters who survived the original movie, Feretti is the one the writers forget exists. In fact, despite dying in the first regular episode of SG-1, Kawalsky makes far more appearances than Feretti (Kawalsky gets Time Travel, Alternate Timelines, Parallel Universes, illusionary worlds. Feretti gets... one brief appearance in the first season finale, one mention in season three, and never, ever has his existence hinted at again.) Feretti's lack of appearance was initially attributed to the actor having other commitments, but SG-1 far outlived Brent Stait's role in Andromeda, and Brent Stait has even appeared in Stargate Atlantis as someone who was not Feretti. He's been treated much better by Fan Fic: the show's sheer lack of love for him kept him on the minds of enough fans that he's got plenty of fanwork dedicated to him.
On Stargate Atlantis Weir and Ford did this before being dropped entirely Weir's actress could not return to do any more episodes (despite the writers having plans for a longer arc involving her), so they pulled (another) The Other Darrin on her (the character's 2nd time) before having her Put on a Bus. Ford just wasn't very interesting to them, so despite bringing him back several times he was eventually dropped.
Family Matters: The character of Judy Winslow, especially in Season 3 until she was eliminated without explanation completely in Season 4; she usually appeared only in scenes involving the entire family or to move a plot along. To a lesser extent, this also happened with Rachel Crawford (Thelma Hopkins, who was involved with other projects), despite making several bit appearances from Season 6 onward.
Step by Step: The character of Brendan Lambert, the youngest son of family patriarch Frank Lambert, was given fewer plots and lines before being eliminated completely by the sixth season. To a lesser extent, this was also the case with Mark (the nerdy middle son of Carol Foster), although Mark would continue to appear sporadically until the series ended, usually given one-liners or in family scenes whereby his absence would be conspicuous. These reduced appearances were perhaps in part due to the shift in focus on the female siblings, especially one-time tomboy Al (after Christine Lakin grew in desirability as she became an adult); while many of the remaining stories were 5-year-old Lily (who was born in 1995 and was age-advanced to create "little girl" stories).
When Mash made it to the little screen, several characters from the movie had this happen to them (when they weren't eliminated entirely): Spearchucker, Ugly John, and Lieutenant Dish were all reduced to second-tier status, and all were gone from the show well before the end of the first season. The character of Spearchucker was supposedly written out for greater historical accuracy, as the writers claimed there was no record of African-American surgeons serving in Korea. (There were, in fact, black doctors in Korea, and Spearchucker was based on an African-American doctor Richard Hornberger heard about at the 8055.)
In Kamen Rider Kiva, Basshaa is the only character to not get a specific arc, and never gets a Fever attack when Kiva upgrades to Emperor form. In the next series, Kamen Rider Decade, Basshaa gets his ass blown away effortlessly in each fight—in the second, he doesn't even get a single shot off!
After Richie Cunningham left, his friend Potsie remained on the show in various different roles before getting a job working for Mr. Cunningham. His screen time, however, was greatly reduced and he wasn't present in several episodes because the writers struggled to find things for him to do.
Also poor Lori-Beth, who is a minor recurring character who later becomes Richie's steady girl. When Richie left, she stuck around, getting married to him over the phone, and giving birth to Richie's son when he was away. Eventually, she just stopped appearing entirely unless it was a special occasion, having little way to play off the others.
John Burns had a couple of episodes centered on him early on Taxi, but he became increasingly ignored, being written off after the show's first season. This was lamp shaded in his second-to-last episode ("Hollywood Calling") when John took a week off to study for his finals and practically no one seemed to notice.
Leoben, in the last half season of Battlestar Galactica. He wasn't even used when they needed background Cylons. A deleted scene does imply only one Leoben remained after the civil war began, but it's never stated directly in either cut material or in on-screen canon.
He does get a larger role in The Plan, the feature length episode produced after the main series had finished. It was, however, set before the end of season 2, so quite what happened to Leoben (and the other Twos) is never addressed.
Once Yutaka Ishinabe, Roksaburo Michiba, and Koumei Nakamura retired from their positions as Iron Chefs, they were demoted to extras, only appearing for specials.
Micah in Season 3 of Heroes. He goes from being a series regular to only appearing in a few episodes of Season 3 after his storyline was dumped from most of the show.
Since Heroes has Loads and Loads of Characters, every single series has demoted at least one of them to extra so far. And completely written out a couple of others.
The Haitian in particular suffered badly from this. Never a prominent character, by the final season most of his appearances were reduced to a single scene every few episodes. On at least one occasion he didn't even get a line - rather, he simply stood in the background to imply that the good guys would use his power in some way (not that they ever actually showed it happening).
Kim Bauer after Season 3 of 24, although as she was The Scrappy in the first three seasons most fans didn't seem to mind.
Much of the main cast from the first 4 seasons of The West Wing were Demoted to Extra as the presidential race took precedence over the running of the White House in the latter seasons. Of particular note, perhaps, is Toby, who was summarily dismissed by the president, and appeared in less than half the episodes of the final season, and those for very small amounts of time (despite Richard Schiff being paid for all of them...).
Kay Howard on Homicide after her promotion to Sergeant, especially in Season 5 where she does nothing until the season finale, after which she's Put on a Bus. To a lesser extent, Munch from Season 4-on and Bayliss for most of Season 7.
Jack McCoy on Law & Order after getting promoted to DA. Interestingly, this was at Sam Waterston's request—when Fred Dalton Thompson left, Waterston campaigned pretty hard for McCoy to get the promotion.
In the first half of the first season of Law & Order: Special Victims Unit Eliot Stabler's four children were regular characters, however after that they became infrequent recurrers showing up about once season. Two of his daughters have not even been seen since the seventh season and between that and the first they had about 3 appearances each. This oldest son (who is the twin of one of the daughters) has remained showing up about once a year and later had A Day in the Limelight, and his middle daughter had a subplot about her being a bipolar thieving drug addict.
The entire rest of the supporting cast on SVU has fallen victim to this. Most noticeably are Munch, Fin, and Huang, who used to get entire subplots in every episode and now don't even appear in half of them. Cragen might have a scene or two, and Liz might have a line per episode. Nowhere near the screentime of Stabler, Benson, and the ADA (Alex, Casey, or whoever the rotating one may be).
Carrie Ingalls on Little House on the Prairie, who went from a major character in the first four seasons, to walking scenery during the next four. She was lucky to get a line once an episode... then once a month... then once a season.
In Robin Hood both Much and Allan-a-Dale, who were hugely important characters in seasons one and two, ended up being lucky if they got three lines each in season three. They were shunted aside in favor of Kate.
The original version of Survivors did this with several characters as the focus of the show shifted from season to season. Jenny in particular started out as one of the principal characters, faded away almost completely, then came back to star in the third and final season.
On Community John Oliver's character, Prof. Ian Duncan was initially featured in early promotional material as the main professor character in the show, but after a handful of episodes has essentially disappeared, with Senor Chang (Ken Jeong) essentially taking his place. Oliver did return in the first season finale, and took over as replacement anthropology professor in the next season. Despite being featured more than he was in the first season, he only turns up in a handful of Season 2 episodes.
The showrunners wanted him to appear more, but Oliver was unwilling to move to California (where the show is filmed) due to his obligations to The Daily Show in New York, so his part was pared down.
Jimmy McNulty in season four, going from being the defacto star in the first three seasons. The actor playing McNulty wanted some time off to do film and theater and spend time with his family, this was explained in series by him deciding to leave the Major Crimes Unit after his adversary was killed before he could take him down, becoming a patrolman and mending his ways while he was at it. He got off the bus in season five, though.
In the fifth season of HBO's , many of the supporting (and even major) characters from the previous seasons appear during several episodes of the series, largely as a reminder that life in Baltimore keeps on going, even when the major plot arcs end. Roland Prezbylewski, the other three main child characters from season four, Cutty Wise, Nick Sobotka, Major Colvin and several others show up in different scenes. More notably, however, various characters who were glimpsed in previous seasons also return, including Clay Davis' limo driver, several dock workers (who are now homeless) and former members of Avon Barksdale's gang who were prominent in the first and second seasons.
Similarly, since the fourth season dealt with the school system, this inevitably led to less focus being spent on the actual police force; which was the forte of Jimmy McNulty (arguably the show's main character). He's even absent for an episode.
Since Season 4 of iCarly started, Freddie's mom Mrs. Benson, has been used sparingly. It's possible that Gibby being promoted to the main cast is the reason for this.
During the first season of Beverly Hills 90210, Scott Scanlon is David Silvers' best friend. As David becomes accepted by Brandon and Brenda's friends, Scott is pushed into the background since he doesn't fit in with that group. He eventually develops a liking for country music and accidentally kills himself in front of David while playing with his father's gun in a second-season episode.
Happened to a lot of characters on Degrassi: The Next Generation, because the show has Loads and Loads of Characters (and they can only focus on so many of them). The most notable example would probably have to be Toby Issacs. He was a major character in the first 2 seasons. Come season 3, he became more of a background character and his screen time was significantly reduced (only getting a total of two major story lines from seasons 3-7 while it seemed like some characters would have a new story line every other episode). He was even removed from the opening credits during his final 2 seasons, before being Put on a Bus for good in season 8.
With the second retool of 3-2-1 Contact in 1986, the seasons 2-4 cast was relegated to a few cameo appearances.
Two of the most-seen recurring villains in the first season, the Raiders and the Home Guard. The Raiders' lack of presence is due to a number of major defeats they suffered in the same episode where the Shadows are seen for the first time, though they begin to appear again later on. The Home Guard get one mention in season 2 before not being seen or referred to again.
To a lesser degree, Lou Welch, one of the low-ranking security guards on the station, gradually disappears in season 2 as Zack Allen begins to get more and more screen time.
Space: 1999: In season 2, Sandra Benes went from being one of the senior staff to barely having any lines when she appeared. That is, if she was even supposed to be the same character. Oddly, she was called Sahn during season 2. Her original role seemed to be given to another Asian character named Yasko.
Especially in Season One, Merlin had a habit of taking the names of reasonably significant Arthurian characters, like Pellinore, and giving them to random knights, who were then killed.
In series four and five, Gaius is given virtually nothing to do except provide exposition and exist as a sounding board to Merlin. True, that was always his role, but he was also given several character-centric episodes in the past. This was not necessarily a bad thing, as the introduction of the knights of the Round Table at the end of series three meant that the show was juggling Loads and Loads of Characters, and one of the themes of the season was passing on the torch to the younger generation.
Gwaine became this in series four and five - oddly enough, he got more screen-time and characterization as a guest star than as a series regular.
Jorja Fox on CSI: Crime Scene Investigation: Sara left, then came back, but though she's credited as a regular, she doesn't appear in every single episode.
In The Tomorrow People, the character of Tyso Boswell was introduced in season 3 with great fanfare, along with several quirks that allowed for strong character and story development (a close friendship with Stephen, claustrophobia, extra-sharp senses, a possible growing-up arc, his Romani backstory, etc). However, after Tyso's second serial World's Away, many of these facets were dropped. Come season 4, Tyso was only just saved from obscurity with the occasional line of dialogue. He was dropped from the series altogether at the end of that season, leaving fan fiction writers to fill in the blanks later on.
JAG: Andrea Parker was the female lead in the Pilot Movie, but wasn’t liked by the network so she was only a recurring character in a few first season episodes and in one season six episode.
Happened to Yvonne Hudson on Saturday Night Live: She was fired along with most of the ill-fated 1980-81 cast, but hung around as an extra until 1984.
Beat the Clock is a rare example of both Ascended Extra and Demoted to Extra. The 1970s version was originally hosted by Jack Narz, with Gene Wood as announcer; Narz stepped down, with Wood taking over as host and Nick Holenreich announcing. A later revival from 1979 to 1980 had Narz as the announcer and Monty Hall as host.