You have to modify the textures themselves and then create new unit cards so they appear correctly in info/review panels.

I'm about to try it myself (convert Greek Hoplites to Macedonian faction); from what I understand, you need 3D Studio Max with the .CAS plugin to edit the models/textures, and you have to unpack the unit card files from the pak folder to edit them. I don't expect it to be overly difficult since all I'm doing is changing colors...assuming it works, I'll be back with a step-by-step...