My set of rules states

Origin in area 1: DONE Reservations are made X days prior to departure of first flight 7days, ticket must be issued 7 days prior to departure.

Origin in areas 2 and 3: DONE reservations any time, ticket must be issued 1 hour prior to departure.

What you appear to be encountering is a D class restriction for purchasing that as a BKK to HKG return in D class, not a D class DONE restriction. Most D class will disappear 2 or 3 weeks before the flight to stop business persons from being able to buy them at the last minute. However, DONE is not like that and agents can still book against D until the flight departs, if available. By issuing later, you risk losing the D allocation on that flight and travelling Y with a waitlist D. If they do not release D and there are spare J seats on departure, they might keep you in Y. It has happened to me so often...

I would suggest the polite enquiry route and check on the D withdrawal policy of CX. What a pitty that there are no QF or BA codeshares on this run for you.

spotwelder.