Question for the Collective Intelligence:

I am trying to find a sci-fi/fantasy book I used to own. Sadly, it's been many years since I last read it and I can't remember the title of the book or the author. The plot is basically this, as best as I can remember:

Kid gets chosen as a sacrifice to his Gods.
Gods are vampiric aliens (they eat souls, at least, and possibly drink blood too.)
Turns out, kid is special. Is either of their bloodline or at least closer to them than to the other humans.
When kid is "eaten", his mind manages to jump into a nearby body... a goldfish, I believe, or something similar.
Ends up taking his place among them, and I believe saving them from dying out.

The really interesting parts of this book center around body-modification: the aliens vary vastly in physical form and put a great deal of effort into customizing their bodies. It's also a really good look at truly alien mindsets/intelligences.

So, does anyone have any clue what book I'm talking about? I've tried some searches, but frankly I just don't remember enough to really find it that way.