A phone call to AA should be the easiest way since they will have all info on hand. They should just make the change and send the info on to QF. You could also phone QF and request the change but they might refer you to AA if their agent can't see the reservation.

In any case, you shouldn't need to make it "open" - you could just choose any date and then when you know exactly what you want - then ring again and get the desired flight / date.