You can also do it at the boarding gate, provided you’re at HKG or an outport with decent staff (eg LHR). If so, I’d generally wait until 20 mins before take off and aim for the senior member of gate staff to do the switch immediately prior to boarding.

Word of warning - if switching to credit on CX and you’re on CX metal there’s little risk, but if crediting elsewhere (eg BA), you run the risk of some “stickiness” in your PNR (ie it keeps the record of your OWE that you don’t want to credit to), such that your other OW membership may decline auto credit and you’ll need to raise a manual / retro claim, and at that point you’re up to the goodwill of the crediting airline. I’ve had this before switching my OWE (I’m CX DM) to my BA status before for crediting purposes.