Retcon

Retcon stands for Retroactive Continuity. A retcon is what happens when canon from earlier in the work is changed later on, and the new information renders the old canon uncanonical.

Retconning is a common problem with comic book continuums, and makes dealing with comic book badfic hard due to the problems of everchanging canon.

Fanfic writers cannot retcon canon, but they can retcon their own work.