I would think it is because switching to the studio from a 2BR is a new reservation. It would be unfair that you automatically get a studio especially if the home resort priority was used to book the 2BR and later the studio is booked at the 7 month mark without giving other members an equal chance at getting the studio after you cancelled the 2BR. Even during the home resort priority period you would be blocking other owners for your resort the change to get the same studio in a reservation. Otherwise people would be booking a 2BR lockoff just in case the family or friends come with them for the vacation.