That's exactly what we had to do 2 years ago at WDW.....the first night we were there was not in Value season, but the remainder of our trip was, so we essentially had 2 reservations. We didn't have to switch rooms, however I do recall having to stop in at the desk to get new room keys - If I recall correctly the keys were activated based on the length of stay...and technically our first *stay* was only the one night. It wasn't any big deal...it only took a few minutes!