retcon

"Short for 'retroactive continuity', retcon refers to the act of changing or adding to historical 'facts' in a work of serial fiction.
...
The most recent Star Wars movie trilogy also served to fill in some of the gaps in the story. Superman Returns (2006) pretended that the awful Superman III and Superman IV never happened - which is no doubt exactly what the producers (and most critics) would have preferred. Fittingly, the term was first used by Superman's publisher, DC Comics.
...
It has been used recently to describe more serious history - Like Keith Windschuttle's controversial take on native Tasmanians (which suggested that, despite what everyone thought, they weren't massacred by white settlers after all). It's also called revisionism, but retconning sounds snappier and less academic."
- 'FYI' in GW, Mark Judder, 23 Feb 2008.

It's also a very difficult word not to misread (at least for me). ;-)

Retroactive continuity. In serial fiction, copying a previous scene in a later episode, but changing or clarifying details to fit with the evolving storyline. It creates contradictions and holes in the story canon but eases the writing of new plot points.