=== Change UA-codeshare, non-UA-metal segment to UA metal: A ===
Tried this; didn't work. Had a UA codeshare AC metal, UA-issued ticket 9 PM EWR-YYZ. I got to the airport a little early and went to the UA premier check-in desk to ask to switch to the 8 PM EWR-YYZ flight on UA metal.
First they stalled, acting as though they had no idea what I wanted. Then they said they'd have to re-issue the whole ticket and contact air Canada. They they started talking about charging me a $250 change fee (or something like that).
Finally, I just rolled my eyes and walked away and stuck with my original flight.
Maybe it would have been better to speak to someone in the UC or on the phone - I don't know.