In my experience there is absolutely no assurance that any award seat will go back into award inventory, and they very often do not whatever the apparent state of the flight.
You take your life into your own hands cancelling in the hope of being able to re-book.
You can never assume anything with the BA system. Indeed it is so bizarre, illogical and indeed perverse, that I have managed to book a CW award seat on the day of travel on a flight which turned out to be oversold in CW, so got a nice op up to F. So the peculiarities of the system can sometimes work in one's favour