As I said, there is domestic first, just domestic business. I'm looking at a UA codeshare ticket I have this week and it's book in AC A class and it is in their domestic business cabin. I would check what kind of seat assignment you can get to confirm that you're in the front cabin but I'm 99% sure it'll book right in there and you don't have to worry on any kind of upgrade.

Interestingly enough, on the UA website in my reservations it shows as AC first, which is probably the source of the confusion for the agent.

If you must contact Air Canada, note that hold times on the general 1-800 number for non-elites often exceeds 30 minutes. Go to their webpage and look up the number for their Hong Kong offices and call there. I'm always on and off the phone in a matter of minutes.