If you request an upgrade to J and your upgrade does not clear Air Canada generously assumes that you want an upgrade to PE. Anyone specifically requesting an upgrade to PE will be ahead of you, but it then goes according to the regular ranking process for anyone who didn't make it into J. You get docked the e-Upgrades to PE.
You do have the ability to decline the upgrade. The risk is that if you don't do this proactively with the gate agent they will upgrade you by default and may give your carefully chosen Y seat away to someone without pre-assigned seating (i.e. "GTE" on their boarding pass) so that when you tell them you want back into Y you get a middle seat at the back of the aircraft.
If you don't want to be upgraded to PE, you had better tell the gate agent before they start clearing upgrades that you only want an upgrade to J and if that fails to clear you want to keep your current Y seat and not be upgraded to PE.