That raises an interesting legal question. Suppose you write, say, a Harry Potter fanfic, then change all the names and maybe an identifying detail or two. You publish it like that. Then you put up a website with a "key" to all the characters and such: Jack is actually Harry, Mr. Thomson is actually Snape, Wizard Rugby is actually Quidditch, et cetera. Does J.K. Rowling have any legal recourse?

Of course, you'd probably piss an awful lot of people off if you actually did this, but it's an interesting intellectual exercise.