One of Megamiles posts from January:

The code-share thing leads to some weird paradoxes. Last week I flew SFO-YYZ. The flight was operated by Air Canada. My ticket was issued by United and showed the United code-share flight number. Since I'm an Aeroplan Elite, I have AC-issued North American upgrade certificates.
So, 48 hours before, I called AC to wait-list for an upgrade - and I was told they couldn't since "officially" I was considered a UA customer. I then called UA to wait-list - and was told they couldn't upgrade me since the flight was a code-share operated by AC!

Anyways, the gate agent was quite friendly and had no trouble accepting my certs - and I think an upgrade on a red-eye is something we can all appreciate !