Provide the ability for the user to select a different room type on the 'Assign Room' screen. If the newly selected room type results in an overbooking situation, ask for an overbooking override before making the change. If the newly selected room type has a different rate than the current room type, ask the user whether or not to retain the current rate or change it to the new rate.
I agree, this would be a valuable asset to have. This would also be beneficial for moving a guest from one room type to another for reasons such as in house moves (maint issues, guest request, housekeeping issues).