Summary Spinoff shows often change canon to develop new storylines and characters, but usually not enough to bother viewers. Young Sheldon and Star Trek: Discovery made significant changes to their predecessors' lore and storylines, which divided fans. Better Call Saul faced the issue of aging actors but stayed true to Breaking Bad's plot, while Legacies took liberties with The Vampire Diaries' canon.
While most TV spinoffs build on the plots of their predecessors, some stray so far from their original show's story that they completely break their canon. It is hard to keep the story of a TV show spinoff straight. After all, all spinoffs need to both continue the story of their original show while simultaneously fleshing out new characters and expanding the world of the series. This is often challenging, and TV spinoffs can sometimes understandably end up contradicting their predecessors from time to time. However, the extent of their canon changes usually isn't too dramatic, and typically these alterations aren't enough to annoy viewers.
For example, while Frasier might not have felt much like Cheers, the spinoff didn't completely rewrite the story of its predecessor. In contrast, Young Sheldon changed The Big Bang Theory's focus so much that the series ended up rewriting pivotal moments in the main character's backstory. Similarly, Better Call Saul stuck closely to the story of Breaking Bad but was unable to avoid messing with the show's chronology due to the actors aging. Other spinoffs simply decide to change the canon of their predecessors entirely even when this wasn't unavoidable, as evidenced in the wonky timeline of The Winchesters or the bizarre retcons of Roseanne's season 10 revival.
Young Sheldon started strong, dropping The Big Bang Theory's hang-out show formula in favor of a nostalgic family sitcom style. However, the spinoff didn't stick to the established backstory of Sheldon Cooper for long. Sheldon's childhood bullying was largely retconned, as were his father's biggest character flaws. Meanwhile, Young Sheldon dropped more than The Big Bang Theory's laugh track when the show made Sheldon's mother Mary a much less bitter, antagonistic figure than she was in the earlier series. While all these changes made the sitcom a warmer show than its predecessor, they also fundamentally contradicted the established lore of the franchise.
Few shows have messed with established storylines and lore as much as Star Trek: Discovery, despite how infamously important canon is to Star Trek franchise fans. From the existence of the Spore Drive to the absence of Spock's brother to the presence of his sister, Star Trek: Discovery did plenty to rewrite the history of the franchise's earlier outings from its first episodes. This was before the series even reinvented the appearance of the iconic Klingons, a choice that left the series feeling like it was set in an alternate timeline. Despite all the show's good qualities, these made Star Trek: Discovery tougher to invest in.
While Better Call Saul's Breaking Bad prequel storyline did fastidiously stick to the plot of the original series, the show couldn't avoid one inevitable issue. In terms of story, Better Call Saul was an ideal prequel to Breaking Bad. However, it was impossible to hide the fact that the returning actors were very clearly older than they were in Breaking Bad even though their characters were younger. This was especially obvious in Aaron Paul's cameos, although it was also forgivable considering how tight the show is everywhere else. That said, the problem could prove jarring from time to time when Better Call Saul was otherwise a darker, even stronger series.
The Nickelodeon shows iCarly and Victorious created a crossover spinoff where the former's Sam and the latter's Cat became roommates in Sam and Cat. However, the spinoff show immediately threw viewers when it started by accidentally retconning an existing meeting between its heroes. Sam and Cat meet for the first time in the pilot of Sam and Cat, but technically the two should have already known each other from the "iParty with Victorious" crossover. While this might seem like a nitpick, the fact that the pair shared plenty of screen time in that earlier crossover outing made it tough to excuse.
When The Mandalorian began, the show was embraced as a worthy addition to the ever-expanding Star Wars universe. However, by season 3, The Mandalorian's retcons made its timeline impossible to fix. Even though showrunner Jon Favreau attempted to clarify when the events of The Mandalorian took place in the franchise's complicated chronology, he was consistently contradicted by Lucasfilm. Everything from Din Dijarin's reunion with Grogu in The Book of Boba Fett to the sheer pace of Grogu's training in the Force was complicated by the movie timeline clashing with the events of the show.
Roseanne's 2017 revival undid Dan's death at the end of Roseanne season 9 since John Goodman's character was a fan-favorite supporting star in the sitcom. However, this meant that the show had to retcon the entirety of Roseanne season 9, turning the events of the season into the plot of an unpublished novel that Roseanne wrote years earlier. While reviving Dan did undo Roseanne's biggest mistake, this was complicated when the revival then went on to kill off Roseanne herself. This allowed the series to become the spinoff The Conners, meaning this is a rare case of a spinoff contradicting canon before the spinoff technically began.
The Supernatural spinoff changed the backstory of Dean and Sam's titular parents. In Supernatural, Dean and Sam's parents had a tragic past involving the temporary death of the Winchester patriarch John and their mother Mary's subsequent deal with a demon. However, in The Winchesters, the pair were depicted as a pair of demon hunters decades before John even discovered Mary's demon hunting in the Supernatural timeline. This was messily justified when it was revealed that The Winchesters took place in an alternate universe, but while this justified the twist, it was hard for viewers to get invested in a version of the family's story that only existed in a different timeline.
The Vampire Diaries spinoff Legacies constantly rewrote the canon of the earlier show whenever it was convenient. Before the series even began, it didn't make sense that Legacies took place in Mystic Falls when this setting was stripped of its magic years earlier on the original show. Later, Legacies made sire bonds less powerful forgot the connections between prison worlds and the Gemini coven leader and generally disregarded the complex rules of the franchise's magic system to allow for more experimentation. While this facilitated some inventive stories, the inconsistency also meant that Legacies often felt nothing like its predecessor The Vampire Diaries.