We had that same issue come up with this upcoming vacation. We booked bounceback last year with
free dining, but we had to change our dates. For 8 nights we get the free dining and the first two fall outside of the free dining dates. So we booked two ressies, had it noted on each ressie so HOPEFULLY we won't have to change rooms. They say they can't guarantee this, but I don't see why they can't.
Anyhow, our first two nights, we paid for the dining plan and got our 10 day tickets on that. The Free dining starts after that and we only have a one day base ticket to make it a package. The CM suggested that because if we did the 10 day tickets on the free dining package, those tickets wouldn't start until the second package starts.
It was complicated at first, but it's all worked out now. Hope that helps.