I suspect QF will swap the QF10 departure slot at LHR with EK for an earlier slot. EK will sell seats on the earlier QF flight, but gain the more lucrative evening slot.
EK has departures at 0910, 1335, 1630, 2015, 2200, so QF may want one of the two earlier slots, which would mean arrival at MEL early or late evening.
Of course EK needs an arrival slot to match, but arrival slots late PM at LHR are not impossible to get.