Need help with modifying monsters

[The chaser]

Let's see:

By what you are saying, I guess it would be something similar to putting Alexander's head into a grunt model, but, to do this, there should be a lot of editing:

First, you'd need to cut off the grunt head and put Alexander's. Then, you'd have to edit the texture itself, so the Alexander skin matches the grunt (or inverse). Then, you'd have to work on the UV map (something that could give you a lot of headaches). And after this, I wouldn't be surprised that animations had problems, so yeah...

Remain that to get something with Blender animations work you need the updated editor, the AMFP one.

About Mudbill's choice: Remain that a billboard is something plain, and it will look just like if the grunt wore a mask (not a realistic choice).