I have never used the bounce back offer but am thinking of doing so. Can they be used on an existing future reservation or only on a brand new reservation made at the time?
They are provided in hopes of enticing you to come back next year, so the implication is that they are meant to be used for new reservations. I imagine it comes down to the CM who takes your call.
Keep in mind the bouncebacks are usually good for a specific date range, and that is far enough out that there would be no penalty for outright canceling a reservation if it can't be applied to your pre-existing one.