I had an aeroplan ticket to UK through montreal
I had it all in biz except YYZ-YUL
I went up to the gate agent 1 hour before boarding to inquire if she was doing upgrades now that the flight had closed - she told me I was at the top of the list.
I'm only E, i'm never at the top of the upgrade list so I'm assumed that the aeroplan ticket trumps status.
This makes sense to me as you've paid for J, you're just waiting for your seat to clear - but i'm sure other people have different opinions.

Edit:
My mother has also been cleared at the gate for a J seat, she has no status at all
I'm pretty sure it was LHR-YHZ