Initial questions have already been answered well, i.e. you can't combine these bookings and AA is not going to check your bags through.

AA does have a policy of protecting separate tickets as long as the other ticket is oneworld, however, as mentioned above getting this correctly enforced can be very tricky at a foreign outstation like YUL and when the other ticket is not on AA. I personally would not rely on this.

Since AA changed the schedule, do your homework and look up other options that get you to YUL earlier. Then call AA and ask to be rebooked on whatever you find due to their schedule change. If you're on the only nonstop, then these other options will obviously involve a connection. Alternatively if nothing else works you can cancel your AA booking for no fee and book a nonstop on another carrier like AC, DL, etc.