A player model consists of two elements ingame: The actual 3d model, which gets assigned at start depending on which kit you spawn with, and the kit geometry, which are accessories like helmets, ammo pouches, holsters etc. There is one of these for each kind of kit. So if you spawn as a scout and pick up a rifle kit, you get a new helmet and other accessories, but your basic player model doesn't change. So you can't change from a Wehrmacht uniform into an old man or something like that.