I don't understand why this is difficult. Call QF reservations and ask for a reservation in D on the appropriate flight and quote the ticket number to them. Ask them to cancel the reservation on the earlier flight. This is how it works with CX, AA and BA. I must admit I haven't tried it with QF, but I can't see why it would be any different.

Or get a tame travel agent to make the new reservation for you and enter the ticket number. In this case all you need to do is make sure that AA cancels the previous segements before you no-show them to avoid the rest of the itinerary being cancelled.

Yet again, I say that if your plans are less than 99% certain, get the ticket issued open-dated and keep the reservation(s) separate. There's no harm in splitting the trip up into multiple PNRs as well just to minimize the damage from any cock-ups.