Changing the player control class, playerpawn, in mid game is almost impossible. Its setup to be a constant once the games starts, so code wise you can't change it. However, this can be modified if you have a hex editor, and you can change some stuff around then code it so you can. This does lead to a horrible mess, as a lot of setup code is executed when you join a game, and changing playerpawns in mid-game wont run that code again. Its a horrible mess, so while its possible I doubt we will ever see it with UT.
UT2k3 on the other hand has made a lot of interesting modifications, so we can expect to see changing models mid-game a standard feature in future Unreal engines