How do I make head fixes etc for class swaps (1.12.1)

[2012]

I know some very basic stuff such as switching models etc with the help of Warcraftstudio, however they always turn out a bit disorient looking. Meaning perhaps if I were to do a human to undead switch the human skin would be covering the entire undead body making it look really odd. The face would also look very weird, sort of a combination between human & undead.

Shortly speaking I get the model switch to work perfectly it's just the textures that get messed up. Any advice?