I'd argue that the biggest decision should be whether to go more literal and "cover" the physical transformation, or to be more metaphorical and express his emotional transformation. I've seen it done both ways; any werewolf movie since "An American Werewolf In London," and any number of other horror films, do the latter, and usually the ol' vegetable violence tricks pay off the best here. Going metaphorical, yeesh, could be anything, but your hands aren't tied to being literal anymore, and your choices can mesh better to the emotion of the character and/or even the key and timbres of the soundtrack, if any.

Check out Jacob's Ladder. Excellent use of sound to sell a very scary visual effect. There's a sample around 1:50 on this YouTube video.

Dissect the transformation scene. If there are obvious physical changes like muscle contractions, fingernail elongation etc. , it is usually easy to cover it with some organic stretch, squash or pinching sounds.

If the viewers are seeing only before and after versions of transformation, than it is up to your imagination and any surreal sound design suitable to the needs of the scene would do the job.

From composing point of view I would suggest using dim5 chords (diminished 5ths) This harmonic distortion supports "something bad" feelings of a particular scene.

With all due respect to: