When my family of 4 goes we always have 2 rooms(DD 26 and DS 14 travel with DH and I)DS and his family(also 4) who live two hours from Orlando sometimes come and join us for a night or two while there. We never know until the day before they come if he can get time off work or not to join us, so we just add them to the room then. I go to the front desk the morning they will be coming(they always come in late) and the front desk CM "changes the room mix" We have not had a problem with this so far and have never been charged for the extra adult either.
We leave in a the 28th for our next trip. DS hopes to join us for a couple of days, but he won't know until the 31st if he can, so we will do as before and "change the room mix" when we know for sure.