Choice award night availability is tied to the room types available for awards at that property. If a room type available for awards at that hotel has paid availability, it will have award availability provided you are within the appropriate booking window. Therefore, if you cancel a paid night for a room that is of a type available for awards and the room goes back into availability (i.e., the hotel is not OVERsold), then it should be available for an award night booking. The issue is that you have no way of knowing on your own if the hotel is oversold.