Also known as "retcon", this is a term from science fiction and from comics. As pulled from the notoriously unreliable Wikipedia entry for retcon (at least today):
"Retroactive continuity" was shortened to "retcon", reportedly by Damian Cugley in 1988 on USENET. Hard evidence of Cugley's abbreviation has yet to surface, though in a USENET posting on August 18, 1990, Cugley posted a reply in which he identified himself as "The originator of the word 'retcon'."[5] Cugley used the newly-shortened word to describe a development in the comic book Saga of the Swamp Thing, which reinterprets the events of the title character's origin by revealing facts that, up to that point, are not part of the narrative and were not intended by earlier writers.
Wikipedia conveniently doesn't yet include this post from rec.arts.comics from Steve Simmons in 1989:
Hear, hear! "Crisis On Infinite Earths" solved a DC crisis all right, but it wasn't the problem of having multiple earths. It was the problem of 50 years of continuity complicating their existing characters. DC had been either ignoring the continuity (which led to contradiction and reader frustration) or attempting to adhere to it, effectively hamstringing them ("remember, we solved that one back in '42"). "Crisis" let DC have good reason to retcon characters to whatever extent needed — heck, it practically demanded it.
Implanting false memories can also be useful for the corporate persona, where parts of the corporate origin story get rewritten to suit the current corporate branding. If you don't have strong documentation for the past – or if no one really cares – you can pull together a selective narrative that picks out the parts of the history that highlight the best parts of the present.
Edward Vielmetti's Wordpress weblog 
Retconning things can make things really weird in serial fiction. Here’s a for instance from South Park:
http://eis-blog.ucsc.edu/2010/04/retroactive-continuity-and-south-park-turns-out-that-santa-claus-was-in-the-bear-suit/
For a whole week, we were correct to believe that Muhammad was parodied in a bear suit on South Park. That belief stirred strong emotions and international controversy. Today, it is correct to believe that Santa Claus, not Muhammad, was in that bear suit. The integrity of last week’s episode is maintained; however, a very consequential reality has been retroactively negated. South Park creates an unusually satisfying experience by using conventions of storytelling applied to an unconventional social situation. I hope that RoleModel can leverage these under-explored experiences and (similarly) apply them to immersive and interactive spaces.
I’ve always been intrigued by the parallels between retcons in the published world and how our thought processes work. People want to remember the good stories over the bad, aren’t always great at remembering what actually happened and sometimes fill in the gaps willy-nilly even if they’re not a BS artist, etc. This works even on a grand scale — the retcon where Iraq was who attacked the U.S. on 9/11. Retcons reflect our fundamental sense of reality far better than, say, video cameras.
More retcons in the real world: The Holocaust never happened; The moon landing was an elaborate hoax; and, 9/11 was an “inside job.”
Another variation on this theme is being explored by Eureka (on SyFy) this season, albeit with a slender thread connecting to previous seasons. I’m fairly certain I don’t like it.
The king of all retcons was the show, Lost. That show could have been *called* Retcon for Christ’s sake.