obsession can change the source of the obsession for the obsessee?

That's a very good point! That describes the situation where someone might end up becoming more immersed in the fandom than the original source, someone who says that they like the fandom better than the show/the book/etc.

I guess I was also wondering when people determine the threshold between being a "fandomer" and simply a "fan"? For instance, there's this band that I like; I love their music, I occasionally watch clips of them and even more occasionally, I read fic about them. But I wouldn't consider myself in the fandom though. The line between being a "fan" and a "fan-fandomer" is a very blurred one. XD