Thanks all, sounds like it's something that might be able to be done over the phone, and worst case (if seats are still available) at the airport.
Somewhat related, does anyone know how eUpgrades work for this flight? I assume I would be able to be waitlisted for each flight, and if there were no J seats available YYZ-YVR that wouldn't prevent me from being on the waitlist for YVR-SYD.