Thanks to everyone who helped me try to get from MEL to YYZ.
Looks good. If business class opens up on AC you can call in and UA will put you in business class (assuming the SYD-SFO leg clears
If you can make changes for free then switch to the later flight, and if the earlier flight becomes available in J then switch back.
So I have never actually had a successful Plan B because every time I set up a Plan B, suddenly space opens up in the award I wanted all along!

As you guys remember, I was waitlisted for I on my intercontinental SYD-SFO leg... well, I just checked now right after shabbos was over here in Melbourne and there was suddenly availability in O on SYD-LAX.
So I changed from MEL-SYD-
SFO-YYZ to MEL-SYD-
LAX-YYZ by using a trivial extra 10k miles and the phone rep (from Manilla, Phillipines) waived the change fee even though my itinerary was booked about 36 hours ago.
PLUS... added bonus... I now have the AC connection to YYZ booked in business! So everything worked out the best possible way. Attached is my new itinerary.