I've also paid for the first night anticipating what room I will be upgraded to if I am using rewards nights for subsequent nights and have never been asked to move to another room.
If one stay takes several reservations, I will call the hotel and have them combine them, which I think removes/lessens the possibility of being asked to change rooms.
I think your Ambassador status and how frequently you stay at that hotel may make a difference in what benefits you get on your rewards nights.