AA is definitely going to lower their transborder fares, because CP is already codesharing with AC and the AA planes are horrible.
For example AA flys the bumpy Saab 340 prop planes from LGA to YUL, which takes 2-3 hours, compared to AC's Airbus/DC9 which takes 1 1/2 hour max. Customers would obviously prefer Jet aircraft over turboprop planes, especially when prices from AA, AC and CP are the same.