The ticket does not need to be reissued but does need to be revalidated. There is no OW fee for revalidation, but the various OW airlines have started to impose fees for this if they did not issue the ticket originally. So you would have to go to the AA office in HKG for a free revalidation (or maybe RTW desk can do this on the phone), or pay the CX fee at the airport (HKD 300, I think).

Does QF now have skybeds on all HKG-MEL flights? When I last checked (year ago) only CX had all-skybed service into MEL. So the choice might be beds on CX with a stop in ADL vs. no bed on QF but non-stop on a day flight. I try to fly at night whenever possible, to save the daytime for cities.