Retcon

Retcon = Retroactive Continuity 

Refers to a story or idea written to change or add to the already established history of a canon or fanfic. Often, it is performed to smooth out continuity so that there are no gaps or glaring errors in the characterizations or plotlines as new ideas are introduced. Alternatively, it may completely change the meaning of all other continuity and result in a total "reboot" of the work.