I can confirm that, when you cancel a OW award and then rebook it a minute later (about as fast as it is possible to do), most of the seats will be gone. Don't forget some airlines allow waitlisting (e.g. CX for their members) so the released seat will go to the waitlist (even to the lowest status member!) before you have a chance to rebook it. It is pointless to book a OW award if you intend to change it later, and the "no changes" rule is pretty firm (I actually had a route cancelled, no longer operated by the airline, and it still required a "special exemption" to reissue the ticket even though it was now impossible to fly; another time flight was switched to one-class and that "involuntary downgrade" was also prohibited by the rules and required an hour of discussion to get the issued award in place again (the computer wouldn't let the agents do it). So just remember, no changes, not even on the same route, flight number and airline (!).