Since UA does not fly between MEL and SYD the second record locator is for that flight, either QF or VOZ .
I suspect you will incur a change fee for the changing the date. I suspect something similar if you drop the leg (which would probably change the fare as you note you got RTW fare).
is the second ticket just for the SYD-MEL leg?
If the GPU has already stuck for the UG then the agent on the call desk should be able to protect this for you, as long as you are not changing the date for the GPU flights. I would also take a screen shot of the R legs.
Good luck and interested to hear what happens.