You call MS and they will see if there is availability. You will pay a $95 exchange fee. If you make any changes later you will pay another $95 fee. It's really restrictive. It is dependent on availability and if I recall correctly has to be booked at least 2 days ahead of check in, maybe a little longer, but might not be availability that late anyway. You also have the rule of 4 so it cannot be booked if you are in the last 4 months of your UY. However you can book earlier and the stay itself can be in the last 4 months of your UY. I think if Disney is booking the rooms then you can probably reserve on points thru
DVC too.