A couple of separate times I have had to cancel a reward booking with zero current availability and rebook the exact same flight, same as you might need to do. In one case the seats went back into inventory while I was on the phone with an agent. On the other occasion I did it online myself and the seats went back into inventory about an hour later. In both cases the travel date was a long way into the future. Maybe I got lucky, but I suspect that if only the standard four award seats have been booked then if any of them are cancelled they make their way back into inventory provided the flight is lightly booked at that time, which of course most flights are that far out. You are taking a risk for sure, but it may work about for you if all else fails.