Should be alright, he just wants to switch what's already in the game. Basically what people do here with FF7.
Swapping the models wouldnt work, Zack has different bones for his different hairstyles. Animations were never found so you can't swap those to make it look right. The only one you'd be able to swap correctly is FF7 style Zack with his beach stuff, and possibly his blood stained clothes.
EDIT: Well, actually read your post :p
Still, wouldnt work because of the problems I just posted.