AA doesn't fly BWI-YYZ or YYZ-YVR. If the OP was always flying WestJet from YYZ to YVR, which seems the most probable reading here, he can't get rerouted to avoid WestJet now.

My guess is OP is flying BWI to PHL or JFK, then YYZ.
AA doesn't fly to YVR from any of its northeast or Midwest hubs, so seeking a reroute would greatly increase flight time. The only reasonable route all on AA is via DFW, which is still out of the way.