Yeah, at the MLL (and I assume at any Air Canada check-in desk at airports where they operate with a large presence AND their allocated gates) they whip out this really sketchy ID card scanner and ask for your credit card to authorize when requesting an upgrade at the airport. Your eUp dashboard is updated to indicate that $XXX.XX funds are held, although I didn't check whether this is a held/pending transaction on my credit card itself in case the upgrade fails. The charge is posted days later.

EDIT: I had a friend do DEL-YVR a few months back who had to rectify a bag charge because they were changing their FFP at the gate. There was no way for them to resolve the now-outstanding bag fees by switching to a non *G FFP, simply because the gate did not have a credit card scanner. I have no idea if you could even get on the waitlist at this point if you had a co-pay to authorize onto a credit card. Maybe this is why AC will invoice you after the fact, like your friend.

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I'd also like to share an edge case that might benefit nobody here but might be interesting in terms of supplementing knowledge to the context of this thread.

My sister and I had a really petty argument and thus I couldn't stand sitting next to her for seven hours in PY. So I went to the MLL and asked to upgrade her, and only her, to J, co-pay and all. They didn't ask for her consent to verify ("oh wow you're such a good brother!!!", "I wish my brother was that nice to me!!!"). We did the whole credit card thing above, I reiterated that I wanted to remain in PY, and she ended up being the only one on the waitlist.

My takeaways:
1) you can upgrade anybody, as long as you have their name (and seat number? idk), without them knowing
2) you don't need to move or upgrade yourself as a result