I was unsuccessful trying to switch an AAdvantage CX F award the other day even when there was a Z inventory seat available (but not to AA, only to Asia miles.) CX would not make any change to the ticket, even at the in-town check in (about 5 hrs before departure). The agent said I could try standby at the airport, but I didn't want to risk that (I think it wouldn't have worked out as that was the last F seat and it was apparently snapped up a short while later).

So maybe this is something they will so at the airport. But I was surprised and a little disappointed that in-town check in at HKG didn't have the same "power."