It looks like 840 never departed MEL on Tuesday Sept 16.
Usually this aeroplane becomes 870 to SFO but instead they chose to cancel 840 to LAX.
Interesting to know why that choice was made because UAL had to 'ferry' a 777 SFO to LAX to operate 839 LAX-SYD that night.

Perhaps the 777 was more valuable to UAL in SFO than sitting idle for 12 hours in LAX
or
is it easier to put displaced passengers to LAX on DL, VA or QF; and fly the SFO and connecting passengers directly to SFO.
who knows ?