User blog comment:Illusive Dream/Fanon, Fanfiction - Open v.s Closed/@comment-8914761-20130920210422/@comment-4966296-20130920211327

I'm assuming it could be as simple as an application for the change to occur. "Application - Fanfiction Switch", in which, the user's applied character will likely be given review and likely accepted (since said character would then be going to their own private timeline/universe).

This is not to say that a character can not take place in a Fanfiction for a period of time, then leave as well, they are free to do so.

One thing to note however, if you are wanting to join a person's Fanfiction, then it is up to said person to give them access to their universe (and then at that point, it is a CO-OP)