I had a JFK-BKK award for December with a long connection in HKG so kept an eye on the earlier flight to open up. About a week ago not only did space open on the morning CX flght but like you said it was a plane swap and had F availbility so I switched to it. Hopefully it won't change again. Usually it's the other way around book F and it goes away as planes swap to one without F cabin.