I remember the Animorphs books pretty well, and it's not them. There are some similarities-- the possibility of getting stuck transformed is a plot point there too, but it doesn't work quite the same, and there aren't any supernatural elements to the Animorphs books, from what I remember, just alien technology.

I'm not sure how accurate this is, but I also tend to think of the Animorphs books as being aimed at a younger audience (I read them around age nine). The series I'm trying to find is aimed at older teenagers, I think.

Thanks for the reply, though!