You are looking at a routing change, which is 1(b), if done before departure from YYZ.

I would recommend going with MiamiPrep's suggestion, so that this re-routing becomes a change after departure, and hence subject to rule 16 2(b), so you should only need to pay the $125 fee (plus any service charge AA might find). If you have an e-ticket, you won't even need to get it rewritten.

There used to be a rule about international segments, but these don't seem to be in the current rules, and, for a different purpose, USA<>Canada was not deemed to be an international segment, so I wouldn't go there!