actually you can change your class in DCUO
to be clear - you dont have classes there just powers but this is a class (another role: healer, controller, tank, dps) and you can change it by the DCUO cartel coins
(it has another name there but i dont remember).
You just need new items becase you cannot wear tank items when you are a healer etc. So i think it could be done in SW TOR to change advanced class - you just will need new items for this class :P