thanks both christep and Guy Betsy !!
a follow-up question...my original reservation is done for the whole itinerary...now for this SYD-HKG segment, if I just make a new reservation with CX for this one flight only, and do not cancel the QF flight reservation...would the airlines think that I am a no-show for the QF flight, and so will cancel all my subsequent flight bookings ? if that's the case, which airlines do I need to call to cancel the QF booking/segment...QF, CX, or all airlines that I have subsequent segments with later in this itinerary ?