Thanks @
RangerNS for the thoughts.
I've never had to change a ticket due to an itinerary change and there is a roughly comparable (time-wise) AC mainline routing going through YYZ instead of YUL. Would AC change my ticket to that for free or are my only options to stick with the Premium Rouge via YUL or cancel and book with another carrier/re-book with AC?
You might get an obstinate agent who refuses to help you, HUCA. Generally, reports are that they are generous in changing things if an already booked flight has been Rouged.
Unless you care about the extra ~160 miles, routing through YYZ would be marginally superior. I'd probably call in that case, but it would be a 5 minute call to the SE desk for me.