Aeroplan often presents "mixed cabin" award flights where they put you in J for a short connecting leg, then stuff you into Y for the long overseas segment - all while still billing you full points for doing the whole trip in J. It's deceptive as hell.
You can request an upgrade at the gate but don't hold your breath, since available seats are almost always gone by that point - especially if you don't have status with AC or have lower tier status. Strongly suggest you select another booking option that will put you in J all the way through right from the start.