No. If you wait till then, the miles will be credited to CX. You must do so before you present the BP for entry For boarding. If you do so after the beep, basically, CX needs to Cancel your boarding, which will hold up the other passengers.

There is a pecking order on how op ups are done. Do a search and you'll see that op ups can be processed as early as 12 hours prior to departure. So if by the time you get to the gate and then change your FFP then, the few minutes difference will not have any bearing as to whether there is a last minute chance of an op up.