I might be wrong - if you already have a reservation booked you cannot just adjust the it online to add
free dining. You'll have to wait until the phone lines open and have a CM alter your reservation. And I believe they are actually cancelling your old one and booking a new one. In 2014 I had already booked CSR, but when FD became available I had to wait and call in to modify the reservation to add free dining. And, the FD deal at CSR was already booked up and I had to change resorts to CBR. Each resort only has a specific number of rooms blocked off for the FD deal. A reservation doesn't guarantee you the FD deal when it becomes available.
That being said - I suppose you could go online when the deal becomes live, book a new reservation and then cancel the old one later.