I think your best bet, other than to pay change fees, etc., is to ask at the airport at check-in IF there is indeed bad weather in YUL. You might also try at YVR in the AC domestic departures area (there is an AC counter there to help people with various issues). I cannot speak for other airports, but I have found the YQR agents to be very helpful if there is even the hint of a possible missed connection. They are good problem solvers. I recently had a terrific experience at IAH with AC agents who creatively helped reroute me due to a potential missed connection at YYC (it turns out that I would have missed my connecting YQR flight after all so it was nice to have them solve my problem before it actually occurred). For that matter, I had a YVR agent bend the rules last winter as well on a potential missed connection to make sure I got home as soon as possible (in that case, a delayed YVR-YYC flight was going to make the YQR flight a very close call but the YVR agent told me that only YYC got sort out my new flight--which would have been the next morning--if I actually missed the flight. I did miss the connection, but the YVR agent had actually issued me the last seat on the first YQR flight the next morning: otherwise I was looking at spending the whole day at YYC). Ask nicely and you never know what could happen. Good luck.