Everything can be changed! Done for a variety of reasons, ranging from poor ratings to someone leaving the show to network fiat, but basically means everything (premise, casting, setting, tone, writing, general emphasis) can be "tweaked" to take the show in a different direction. Not everything changes; some retools are subtle, some not so much. Drastic retooling runs the risk of alienating the current viewership ("change is bad!"), if any.
Many examples of retooling come between a show's pilot and the episodes made after the series is picked up. Others happen when a show isn't really getting off the ground or is in decline and the creators want to shake things up. When done out of nowhere in the middle of the show, then you've got yourself a Wham Episode. A retool may also be the result of a Post Script Season; the series ends up going in a strange new direction because all the prior conflicts were already resolved, and new ones need to be invented.
In many cases a retool is needed because as it existed previously, there might have been very little room for characters to grow or that the established rules hindered creative stories. In some cases, when the retool is so drastically different, you are asked to accept what came before in Broad Strokes. Quite frequently, a retool will include one (or more) Tone Shifts.
The most extreme form of retool is the Continuity Reboot.
See also: Retcon, Revision, Rewrite.
Pokémon began doing this after the Johto saga, when Ash would leave most of the Pokémon he carried with him at Professor Oak's and travel to the new region with just Pikachu. This is meant to create room on his team for Pokémon from the latest generation of games.
They did it once more for Best Wishes by having Ash catch more than six Pokémon at a time and putting them into rotation.
In addition, they made Team Rocket far more threatening than in previous sagas where they were everyone's Butt Monkeys
Ironically, that is how Team Rocket used to be in the first season. Then they got retooled into the idiots they were.
After five years of success, Dragon Ball was completely retooled in the late '80s: The series became much more focused on fighting 24/7, several of the past characters had their roles reduced or were outright dropped, the series gave bigger spotlights to some of the current supporting characters while often shifting Goku to the background at times, and added a much more sci-fi feeling by revealing Goku was an alien. It's not surprising that these changes in the manga were what lead to the anime being renamed Dragon Ball Z
The original IGPX miniseries was a mecha combat tournament. The actual series is a racing anime.
The original anime adaptation of Ranma One Half originally lasted for eighteen episodes and aired on Fuji TV, but was canceled due to low ratings. The show's crew regrouped, gave the show a retool (most notably, the boys in Ranma's school now are completely oblivious to his transformation into a girl, whereas in the manga, they were fully aware) and relaunched it one month later as ''Ranma 1/2: Netto-hen", which proceeded to last for 143 episodes, two movies and a number of OV As.
The Astérix album Astérix and the Falling Sky completely shifted the focus, theme and and tone of the series, transforming a historical comedy into a science fiction pop culture reference smorgasbord.
Marvel Universe heroine Patsy Walker has gone through numerous retools. Started out as an Archie-style teen comedy, moved over into more straight romance, became a superheroine named Hellcat, became Darker and Edgier, and now is... Just weird. And Awesome. Note that all of that stuff is still technically in-continuity.
By 1970, X-Men was an unpopular series that was reduced to reprinting old material. 5 years later, after getting a new writer, putting most of the old team on a bus (temporarily) in favor of other characters, and dedicating as much time to Character Development as fights, the series picked up a great many new fans.
The Marvel Comics series Thunderbolts has always fundamentally had the same premise (a super hero team (that term used loosely) comprised of villains). However, the exact nature of the team has been changed several times, amazingly with the series lasting over 150+ issues only once being canceled and relaunched once.
Originally, Thunderbolts centered around a team of heroes that were actually Baron Zemo's Masters of Evil in disguise working to gain the public's trust so they could easily overtake them.
Eventually, the team (those that decided to reform and be actual heroes) came under the leadership of Hawkeye up until a short period where all the previous story lines were abandoned and the book was made into a super hero fight club. Shortly after that it was cancelled.
It was brought back soon after around the time of New Avengers' release with a cast featuring some new characters as well as old ones until the Marvel crossover event Civil War.
During and after Civil War the team consisted of more popular villains like Green Goblin, Venom, and Bullseye working as "hero hunters" for the pro-registration side to capture anti-registration superheroes.
During Dark Reign most of the hero hunter team became the Dark Avengers and the Thunderbolts became Norman Osborn's personal hit squad.
After Dark Reign and at the onset of Marvel's Heroic Age the Thunderbolts became super villain prison The Raft's rehabilitation program for super criminals (this time including Man-Thing, Juggernaut, and Ghost among others), under the supervision of Luke Cage.
With #175, the title got renamed to Dark Avengers with characters from the second incarnation of the Dark Avengers joining the cast, still under the leadership of Luke Cage.
Marvel Now: the Raft program got shut down, Red Hulk took the Thunderbolts name for his black ops squad of antiheroes, and the Dark Avengers found themselves independent once again.
In the pages of Superman during 1971, an experimental "Kryptonite-Engine" made to provide cheap electrical power malfunctions, causing all the Kryptonite on the planet to become ordinary iron. Meanwhile, Clark Kent became a TV news reporter while an Evil Twin of Superman made of sand drained him of some of his powers. You can read the full saga here. After the entire story is resolved, the series's new direction was quickly lost and more Kryptonite arrives from space, the only holdover from the storyline (until Crisis on Infinite Earths) being that Clark works at a television news station.
By 1968, the Metal Men were among the Denser And Wackier of DC Comics' output. That all changed in Metal Men #33, which began a Story Arc where the team could not control their increased powers and find themselves hunted by humans, who turned against them. It came to a head in #37, where the Metal Men were finally apprehended and left for dead in a junkyard. Mister Conan salvaged them and gave them human identities so they could continue to help the world in secret.
In the wake of Johnny Storm's death the Fantastic Four has undergone a (probably) temporary change with them becoming the Future Foundation. This has involved them donning black and white uniforms, adding Spider-Man to the team to replace Johnny, bringing Doctor Doom and Mister Fantastic's time traveling father along for the ride and becoming a sort of superhero think tank. Thus far the new series has been well received.
In the late-1980s, Steve Engelhart tried to spruce things up with the 'NEW' Fantastic Four, with Reed and Sue Put on a Bus and replaced with Crystal and the second Ms. Marvel.
DC's September 2011 relaunch stands with one foot in the Continuity Reboot camp and another in the retool camp. Some characters are getting retold origins and backstories (Superman is getting his early days retold, with him now being the first superhero in the DCU) while others are simply getting a change to the status quo (Bruce Wayne is now the only Batman, with Dick Grayson going back to Nightwing and Damian remaining as Robin).
X-Factor started off as a reunion book featuring the original five Silver AgeX-Men, and in the In the 90's, was retooled into a government-sanctioned team of mutant superheroes lead by Havok. Peter David later relaunched the title again in 2005 as a Noir detective title consisting of a bunch of former X-Force and Generation X members.
Titans was originally a superhero book about the now-adult former members of the New Teen Titans. During Brightest Day, the concept was completely revamped, and the book ended up becoming about a team of Anti-Hero mercenaries lead by Deathstroke.
Katy Keene had two revivals, each retooling the story their own ways.
The Punisher spent better part of the late eighties and the first half of the nineties by killing every kind of criminal on the planet. After he supposedly killed Nick Fury in Over the Edge event, he was sent to the electric chair at the start of his new series, only to be revealed that his death was faked by the Geraci family, who made Castle their new don. He still continued fighting crimials, but now at his "family's" interest.
This was followed by another retool, where Castle became an agent of Heaven with celestial weapons.
The Star Trek franchise features several notable film examples:
Star Trek II: The Wrath of Khan retooled the franchise after the mediocre results of the first movie, by bringing back a classic villain and retooling Starfleet as more naval-oriented than it was portrayed in the original series.
The 2009 Star Trek retooled the entire franchise after the dismal box office results of Star Trek: Nemesis. The focus was changed to a different timeline following the (now changed because of Romulan influence) adventures of the crew from the original series.
Star Trek: First Contact radically redesigned the Borg in many ways. The most obvious one is cosmetic, they looked like they were being rotted out from the inside whereas their prior makeup was pale guys in armored suits. They then established the idea of the Borg Queen because without a leader they are really just slightly more difficult zombies. Then it showed that they assimilate people en mass as well as technology, whereas in the TNG "Best Of Both Worlds" it was suggested that Picard was a one time deal. And lastly, partially because of the existence of the Queen, they are shown to have a much greater sense of tactics and strategy to even attempt the Time Travel plot, before they were just overconfident in their malevolence. But there is a reason First Contact is considered the best TNG movie.
After Batman Returns came under fire from parents, watchdog groups, and merchandise-tie-in companies such as McDonald's for being considerably darker, more violent, sexual and disturbing than its 1989 predecessor (as well as not even getting remotely close to equaling its box office intake), director Tim Burton as well as star Michael Keaton and composer Danny Elfman left the series. In their place for the Lighter and Softer (as well as brighter) third movie, Batman Forever came Joel Schumacher, Val Kilmer and Elliot Goldenthal respectively.
And after this direction proved disastrous in the follow-up film, Batman & Robin, the series lay dormant for eight years until a Continuity Reboot retooled the series again, putting as much distance as possible between the Batman franchise and the embarrassment that Joel Schumacher had turned it into, resulting in Christopher Nolan's darker, more realistic, and more grounded Batman Begins, which became the first of a trilogy, The Dark Knight Saga, that is praised as a return to form for the series.
Starting with the third movie, the Planet of the Apes franchise switched from following human characters in the far future to ape characters in the present or immediate future. Burton's remake attempted to return to the astronaut protagonist, but after its poor reception, the series went back to ape protagonists with the Rise of the Planet of the Apesreboot.
After nearly four decades as a knockoff of MAD, Cracked morphed into a "lad mag" akin to FHM or Maxim. This retool was short-lived and the magazine died soon afterward, only to be revived online as the list-heavy humor site it is now.
The British magazine heat launched as an entertainment-focused, hipper alternative to the RadioTimes. Although this was well-received, it didn't do well commercially. A series of quick makeovers saw it repositioned as a more downmarket, gossip and soap/reality-celebrity focused publication aimed mainly at women, and it's now one of the UK's biggest-selling magazines.
Blondie was originally about a flapper and her rich boyfriends. When she married one of them, Dagwood Bumstead, he was disinherited, had to get a job, and lived a life more of the audience could identify with. On top of that, said husband essentially became the main character.
Beetle Bailey started out as a strip about a ne'er-do-well college student. Then, very early in the series' run, the main character joined the army, where he has been for the last sixty-three years.
When Garry Trudeau returned from his 18-month sabbatical, the main characters of Doonesbury left college (and the town the college was in) behind, got careers & families, and started aging in real-time. This caused a noticeable shift in the perspective of the strip (although its political nature never changed).
Although at the very beginning, the cartoon wasn't really oriented towards covering politics at all, being about the college life of its main cast and making this a double example.
Chris Bores of The Irate Gamer had another review show called The Breakfast Rant, but it was eventually retooled into I Rate the 80's in order to broaden its scope.
After being Uncancelled, The Nostalgia Critic changed up the format of the series: First and foremost, his movie reviews now cover any film as long as it is not in theaters. Reviews are now bi-weekly, with every other week devoted to short editorials. Also, two of Doug Walker's cast members from his series Demo Reel, Rachel Tietz and Malcolm Ray, are now regular members of the cast.
Spoofed by an ad campaign which aired between seasons of King of the Hill. The second season ended on a cliffhanger with the local Mega-lo Mart (a Wal-Mart expy) being destroyed by a propane explosion. Four characters, including protagonist Hank, were inside at the time, and FOX told viewers that one of them would die. Ads that aired throughout the summer showed viewers a "behind the scenes" disagreement between Hank and FOX, which threatened to kill him off unless he agreed to allow the show to be re-tooled by moving it to Los Angeles and retitling it "King of the Hollywood Hills." Hank refused, and eventually got his way thanks to Bobby accidentally getting a hold of some compromising photos of a FOX executive. Of course, in reality, there was no such dispute and the writers had always known from the start who they were going to kill off (Luanne's boyfriend, Buckley).
Spoofed on The Simpsons episode "Homer to the Max" (1999). Watching the first episode of Police Cops, Homer is thrilled to discover he shares his name with its Don Johnson-like lead character (catchphrase: "And that's the end of that chapter!"); the next week Homer is horrified to see his character retooled as a blundering doofus (catchphrase: "Uh-oh, Spaghetti-Os!"). He seeks out the show's producers and writers.
Homer: Uh ... so, I just wanna know how come you made your Homer Simpson character so ... Producer: Stupid? [laughs] Well, I can assure you, it happened organically. Homer: It better have!
Another episode featured a RobocopExpy. Homer wanted to watch it before it got retooled. A couple of seconds later the robot (who is also a Father) quit the force and got a job at a fashion agency.
The '80s-'90s cartoon version of Teenage Mutant Ninja Turtles was retooled for its eighth season, going through significant changes in audiovisual style and tone. The episodes after the retool are commonly known in the fandom as the "Red Sky Episodes", since this was the hue the backgrounds almost invariably took. The story itself became darker, with the Shredder going from Affably Evil to total Big Bad and more threatening than ever, and the Turtles becoming wanted by the NYPD for failing to stop Shredder from blowing up the Channel 6 skyscraper.
In the ninth season, the series received more changes. Shredder and Krang were Put on a Bus and replaced by Lord Dregg, who would became the main villain for the rest of the series. The Turtles also got a new sidekick named Carter, and there was also a new subplot involving the mutagen, that turned them the way they are at the very beginning, going wrong, turning them into large mutant monsters.
Like its predecessor, the second Teenage Mutant Ninja Turtles animated series was eventually retooled. During its sixth season, its setting was changed from the present day to the year 2105 via accidental time travel. "Fast Forward", as the season was subtitled, featured a shift in art style (simpler) and in tone (lighter), the abandoning of most of the show's supporting cast in favor of completely new ones. A second, milder retool occurred with the seventh season, which featured the turtles' return to present day, yet another Art Shift, and a new subtitle—"Back to the Sewer".
And then came CNReal. It wasn't successful, though.
An In-Universe example in South Park episode "Whale Whores". When Stan Marsh overtakes Whale Wars reality show, he begins some radical actions to save whales and dolphins, but everybody sees it just a retool of the show.
An in-universe example happens when the show's ratings go down a tiny bit and the executives decide to add a few new changes to the show. Instead of a lab, they now live in a house in the suburbs with their adopted kids (one of them being an UrkelExpy) and a sassy robot.Naturally, Brain immediately quits. It's been suggested that this was written as a response to what the writers knew was coming. The network didn't get the message and the retooled series lasted five or so episodes.
KaBlam! got slightly re-tooled in it's second season, giving new personalities to Henry and June, changing the overall look of the characters, and Art Evolution and new theme tunes for some of the shorts. Also, the jokes were less "random" than the first season.
The show was also briefly retooled in the fourth season. The comic book-setting was pretty much abandoned (with the exception of "turning the page", though they really couldn't get rid of that, as well as the opening and ending themes), the show's TV studio setting was more apparent, the jokes in the Henry and June segments became less random and more "mature", and most of the "classic" shorts skipped a few episodes.
Disney's One Saturday Morning was retooled twice. In September 2000, the original hosting segments from 1997-2000 which took place inside the One Saturday Morning building with live action hosts on the virtual set were axed, along with all the shorts which aired in-between programs (Excluding Schoolhouse Rock which ran until 2001). The new on-air bumpers would feature live-action kids playing in a park (Along with the "1" logo, and in the opening, the cast of Recess, The Weekenders (2000-2002), Teacher's Pet, Lloyd in Space (Beginning in 2001), and Teamo Supremo (2002; replacing The Weekenders), which was also used for the new version of the theme song along with said characters. The new theme song was the same as the old one, but sung by a young girl and was shortened. In 2002, shortly before the switch to ABC Kids, repeats of Disney Channel shows began airing.
The block was then retooled and rebranded into ABC Kids in September 2002, the same day Disney's purchase of the Fox Kids assets following their buyout of Fox Family (Into ABC Family) which was included with the sale. The new motif was that the on-air bumpers had each show's characters interacting in a stadium setting. Due to the retool, every show on One Saturday Morning that weren't repeats of Disney Channel shows were quickly cancelled, with the remaining episodes of Teacher's Pet, The Weekenders, Lloyd in Space, and Teamo Supremo would be dumped off on Toon Disney. The only show to survive the block switch was Recess (Which was in reruns), due to high demand (It was the highest-rated ABC animated show, highest-rated Saturday morning cartoon, and third highest rated animated series in the late 1990s) and ABC wanting to renew the show for another season to add to the initial sixty-five episodes...which unfortunatly never happened. The only new shows to premiere on the block were Fillmore! and various Power Rangers series following the purchase of the franchise. Everything else on the block were repeats of Disney Channel shows, and by the time the block came to an end in 2011, the entire lineup was made up of nothing but Disney Channel reruns.