Datapoint about changing CX flights after the first segment has been flown.
Itinerary was SIN > HKG (stop) > LAX. SIN > HKG in J and HKG > LAX in F. A day before flight to LAX, space opened up on an earlier flight. (Space checked on QF. F5 on EF) Called the line and agent in Seattle picked up. Just asked to be changed onto the earlier flight. Put on hold and about 10 min later, came back and said everything was good to go. This was for 2 separate reservations flying the same itinerary.
CX reservation immediately changed and check-in went smoothly.
Gold 75K here if that may have had something to do with it.
Award space from original flight also went back into award inventory. (Checked on QF)