I agree with the previous poster. Specifically tell the cast member exactly what you want to do. In this case, it should be what they'd do for you automatically, but it never hurts to be very specific.
"Please check for availability for a two-bedroom at BWV on these dates. If a Boardwalk room is available, I want to cancel those nights from my existing BLT reservation, and re-book at the Boardwalk. When re-booking, please be sure to use the borrowed points first, so I can bank the 21 current use year points that will be left over."
If you know you're going to bank those 21 points, have the same cast member do the banking for you. That will ensure he/she did everything correctly with the reservation.
I also like to be right in front of my computer when I'm talking to Member Services. I hit refresh on the member web site until I can see the changes they made.