This is what happened to us last summer...
We were already booked last year for a trip in June when the FD promo came in. I called and basically had to make a new reservation.
Yes, they'll honor the FD and try to keep your current accomodations HOWEVER, like Zip mentioned if the room blocks for that promo are already sold out, you do end up having to pick a different place which could possibly mean upgrading.
Our prior reservation was canceled all together and the new one with promo was booked AND I had to pay all new deposits (we had 2 rooms).
Maybe this was just a fluke, but yes, I had to make a whole new reservation, had previous deposits refunded (2 weeks later) and paid new deposits for our new reservations to take advantage of the FD. Because FD wasn't available at POR we had to move to CSR and into a preferred room.
Just sharing that in a way, everyone is right.