Retconning history is so necessary it had to be invented. The BBC series Torchwood popularized the term for modern audiences, but it was coined back in the 70s and then taken up on Usenet (or maybe this is a retcon) to describe the practice of Comic Book canons to suddenly have whole new backstories for a character.
Retcon stands for Retroactive Continuity, and in Torchwood, it came in pill form for those uncomfortable moments when someone needed to be disabused of uncomfortable truths and had to be induced to believe a new past, with actual events cast in a different light.