It's a bit like District 9 in that regard, I guess. However, the aliens are even stranger. I think they're quadrupedal. Maybe their tongues are long, I don't remember. I also remember there may have been copulation between the transformed protagonist and a member of the alien species.
The aliens have very distinct senses, which the protagonist discovers in much detail. Possibly their sight or something with their tongues.
The events happen on the alien's planet. The point of the story is one of appreciation for completely alien cultures - the humans come as conquerors, and one of them learns to live with the "natives" (once he becomes one of them).
Based on when I read it, it must have been written somewhere in the 20th century, probably before 1990.
It's none of the books on this list: http://en.wikipedia.org/wiki/Category:Shapeshifting_in_fiction
I'm sorry for being so vague, but this is all I can remember.