When I first became aware of the FD discount years ago, I was told by the travel agents I talked to, that you could book your room, and then any discounts that came along, you could just apply them to your reservation. We all now know that's not the way it works, and you have to totally cancel and rebook a reservation but only IF there is availability for that room type, or even in that resort. But you can't get people to believe they just can't apply it to an existing reservation.
I always book the resort I want, because I know I'm going regardless of discounts or not. If something comes along we can take advantage of, I consider it pixie dust! This fall, we are booked at a moderate, during a time that FD historically doesn't include. But if a nice RO offer comes along, we'll gladly jump to a Deluxe if we can, but otherwise, we'll be very happy where we are.