I don't know but I am curious:
1 - are there 2 PNRs on the UA side or 1 UA + 1 for SYD-MEL on another airline?
2 - is it 2 fares RDU-SYD/MEL-RDU + SYD-MEL or a through fare RDU-MEL-RDU?
3 - if it is a through fare, does the rule allow a stopover in SYD?
...and it is not a "schedule change" as that usually refers to when the airline changes the schedule on you.