Originally Posted by bocastephen

I did not have any problem - I was able to check-in online with Air Canada and get an AC boarding pass sent to my phone, and with the better cookies over there, I went to the MLL and was scanned in without a problem. Boarding was no problem either, and the flight went out with empty seats in F.

If the flight was full in F with waitlisted upgrades and I took the last seat, perhaps they would have hassled me, but the UA agent did call the AC ops dept in LAX and asked first if they could take one of their F seats for my rebooking.