I don't think you have much choice. Right now you're booked on that domestic 767 YUL-YVR flight so I understand why you want to switch to the A330. I think all you could hope for is a great agent that is willing to reroute you via YYZ [since you're on a YUL-YVR one segment flight, they might not be willing to do it]. Try in the MLL as well.

The other option you have is to standby for the 5pm ex-YUL. It's a one stop flight under one flight number (Similar to the one you were on ex-YVR). That's a A330 flight and it stops in YYZ. It still looks wide open so you have a pretty good chance of getting on.