Availability is such a poor word to use in this case, but I can not think of any better one to use. Using my reservation as an example, there are already a limited number of rooms since there are only so many floors in the Contemporary, and in turn only half of those rooms face the MK.
So for simple math's sake, let say there are 100 rooms that match that designation, Contemporary - Theme Park View - Main Building. That being said, when they release a deal like this, they set aside a percentage (unknown how it is figured out) of each resort's rooms for the sale. Let say 20% so that means 20 rooms are eligible. If the resort is not sold out, it is a first come/first serve basis. If that designation is already sold out, like mine was, it is a first come/first serve basis on who can change their reservations first.
Cancelling a reservation to rebook will not get you that discount slot if the amount of discounts are already "taken", and you run the risk of losing your slot altogether if they do not put it up right away for you to rebook, or someone else books it quicker. When you want to make a change like that, it is always better to call in and suffer the holds, so you do not lose what you already have.
What frustrates me is we also had a unique pin code available, which released today, and I thought that fed from a different batch of rooms, not the same as the open discount rooms, which are now not available.