Comics themselves have a long track record of forgetting / ignoring things like character histories, established universe rules, and even character deaths. And that was before things like the establishing the multiverse, and those oh-too-often universe-wide retcons known as "crises" in the DC parlance.
Now, while my old gaming group was very much dedicated to the 'death by the rule of the dice', there were instances when certain things would be replayed or adjusted.
For example, the GM forgetting to inform a PC with a set of extra-normal senses (that are always on) about a crucial clue that would've changed the outcome of a situation. Or the incorrect calculation / interpretation of a rule in combat. These -- in my group -- would've triggered a re-roll or a re-play before the events were committed to campaign canon.
These changes could be likened to editorial fiat of comic books, before the book gets published and distributed to the masses.
Next Session Fixes
Similarly, if a cosmic-powered villain run by a bloodthirsty guest GM that ended up killing a PC or two, then one could expect that game to be entirely ignored canon-wise (but experience points retained, of course). Or perhaps certain villainous NPCs were portrayed / played incorrectly -- they would be revealed as fakes or copycats. Or if a PC had played out of character (a rare occurence due to personal reasons), then perhaps the copycat rationale or a mind-control subplot would be hatched -- that the player and PC would then have to endure for a series of sessions until resolved.
Of course, there are some game sessions where everyone -- including the GM -- agrees that a certain character, or series of events, or the entire storyline was an abomination, and never happened. I know I still do that with certain comic book storylines or details -- Cable, I'm talking about you.
A Character Revamp
This could be due to many reasons:
- a new GM who didn't like the old character's build;
- a new ruleset that mandates (or gives an excuse) to refine the current character's build;
- boredom with the current character concept, or an insight into what the player really wants the character to be;
- a new movie / TV series / cartoon / comic book gives a new spin or take on a character archetype and a player wants in on that.
But, like comic books -- this may all be necessary to keep the characters, the game, and the campaign fresh and relevant to the players and the GM for years to come.