The addition of Mexico is likely because that was the language on the CP stickers (i.e. North America: Canada, US and Mexico) and they could be used on the daily YYZ-MEX flight. With that flight now an AC designator, it just brings things in line. My understanding is that upgrades cannot be used on codeshare partner metal flights even with an AC designator, and if AC metal, booking must be with AC not codeshare partner.