[Munchkin]

I have a question about race and class. The rules say at anytime, even in combat, you can say "I don't wanna be a ________ anymore." So when's the next time you can update to something else? My friends and I had been playing additional race/class cards sideways but I guess that's only for items correct (and not once only items if I understand correctly). So if that's the case then would you have to keep other races/classes in your hand and if so would the player have to wait until his/her next turn in order to update to something else?