I used to model edit alot to swap races, and the biggest problem while doing this is that certain races have more features than others. For instance, female Human has 14 skin colors while female Gnome has only 6 colors, so I'm wondering which color do I get when I switch from female Human with skin color number 7+ to female Gnome?
This is just an example, but basically all races have different numbers of skin colors, hair styles, facial features, etc. I'm not saying that this is impossible, but it might require some client side modification too, to add the extra textures or other missing datas.