AC should open the space for you, but UA will have to get them to do it then reissue the ticket. As others have said, AC is not going to make the change directly on a 016 ticket unless it's very close in to departure. Keep pushing with MP.

Rerouting via IAH is a lousy option, unnecessary US CBP (utter gong show without GE as a whole bank of Latin American flights arrives at IAH early in the morning), hours of extra travel and crappy UA or AC NA J product from IAH to YYZ.