The close-in upgrade offer is by no means a reliable to score a bargain. More often than not it’s more expensive than just paying the fare difference.

Aeroplan doesn’t offer *A upgrades by miles so that’s not an option. If you have a way of getting KrisFlyer miles you can try seeing if any upgrade availability pops up.

But to answer your question, the “best” way is to just pay the difference upfront, especially if you really need to fly J.