It is possible to change the character appearance to that of a different gender/class. See
Some more hex offsets... for details, and
Teleporting Anywhere, via hex editor as a reference.
AFAIK, though, nobody has changed the appearance of the main character to a non-main character model. Enemy creatures or NPCs are not going to have all of the animations that the main characters have, so you would at least end up with cosmetic problems if it was possible to switch them.
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