You can cancel the resie on the member site and then book a new one. There is a risk of losing both... you cancel your home resort and in the time you are doing that and going through process to book new resie, the new resort gets booked up. You need to weigh the risk. But to call in to make switch means you have to wait until 9am and the new resort can get booked up in that hour.
You also need to call MS if you plan to buy OTU points to make the new resie. And transfer points don't show up online so you'd need to call if you used any in your home resort booking.