First, you need to verify the cost to do a SDC.

International P fares do not always have free SDC. I pulled up a dummy YTZ-LHR round-trip P fare, and it says "Day of departure, at check-in or at the airport - changes are permitted free of charge and subject to availability.", so it may be possible.

Assuming that's true, you should be able to just show up at YYZ and do this. Don't expect this to be a 30 second change though.

Reservations would charge you change fee ($500?) plus fare difference, and OLCI has never offered me more than a simple segment to segment change (i.e. you might be offered other YTZ-YUL options).