It has been my experience that the agents at the airport have a lot more flexibility than do those on the phone and most websites offer no flexibility at all.
I'd inquire at the airport when you depart.
If that fails, you can almost certainly get this change at no charge on the ground in Canada, although checked bags might be a problem.
A confident and friendly attitude will solve many travel problems.
Way early at the airport and wishing for an earlier flight than the one on which you're booked?
Just go to the counter and ask. If they have seats available, you'll probably get the change at no charge. Done that.
Potentially stuck in the Carribean (really awful, I know) during the northern winter due to weather to the north?
Just go up to the counter, smile and speak calmly and you'll get a seat on another flight to your connecting airport, while the many who rant and rave are put on standby. You can smile at them as you board and wave at them at the gate as your aircraft is pushed back. Done that.
Potentially stuck in Newark (and that really would be awful) because of weather?
Just persist in pushing the agents handling rebooks to find you an interline that will get you to your destination ASAP. Smile and remain calm, but don't accept the first offer of a flight three days later on their airline.
We then got an CRJ home the following day and spent the night in a really decent chain hotel across the field. Had a decent meal and a Guiness or two. The shuttle ride to LGA which involved passing through the Holland Tunnel and Chinatown was also a hoot.
Upgrade to the front of the airplane at minimal cost?
Ask at check-in, and if that fails, ask again at the gate. It is better at check-in, since that saves you the cost of paying for checked bags in the U.S., but a little space is nice either way on flights of anything more than a couple of hours. Done this a number of times.
Bottom line is that it pays to ask a real live person you're looking in the eyes.
They have more flexibility in accomodating passengers than many seem to think.