gate agents are more than happy to change ff at gate, but obviously this should be done after an e upgrade clears. believe it or not gate agents there to help, there's no reason for them to care about ff (unless otp is in jeopardy) and it doesn't take much effort
Technically that is against the rules for the eUpgrade account holder....but it does not specify whether the Nominee can change FF numbers or not.:
If you wish to use eUpgrade Credits to request an upgrade on your itinerary, you will be required to use your Aeroplan Number as your frequent flyer number. If you wish to earn miles or points with another program, your eUpgrade request will be cancelled.
https://beta.aircanada.com/ca/en/aco...s/request.html