CX first class on the 747 is far and away superior to the QF product. BA is also better than QF, so I would try and change to CX or at least BA.
One issue is that currently CX uses a A340 on some days to connect SYD to HKG for the late night London flights. Although they are supposed to get the new seats they currently do not have them. If that would be the case on the day you fly I would switch to BA. You should be able to ticket BA using a QF flight number thus avoiding a service charge if you have already been ticketed.
rich