This exact same thing happened to me recently. Member services were not helpful... I wanted 10 days at Aulani, and was "holding" some of those days with limited availability with my current membership (didn't hold all 10 days as I would have had to borrow points and then they would be stuck in this use year). Because my new membership (with expiring banked points) has a different use year, when I finally closed on the contract and got the points in my account, they would not reallocate the points, and instead only would book the available days. This left me with 3 days booked on my old contract and 7 days booked on my new contract with the banked points. They made me waitlist those 3 days with the new contract, so when someone else cancelled, thankfully my waitlist came through and then I could cancel those days that were now double booked on my old membership. They said because it was a different use year/membership, there was no guarantee they could "grab" those days when I cancelled, and there was a chance I would lose the booking. I complained and asked for a supervisor, and she said they may have been able to do it if we were direct purchasers, which is BS... My husband was going to call back and lose it with them if the waitlists didn't come through quickly.
EDIT- oh and it is my understanding that if you change which points you use for the booking within 30 days, the previous points you used to hold the reservation will become holding points