You will have to deal with UA on this one, as it is their ticket. AC will only help you if there is a problem on the day of the flight at the airport. UA should protect you, should their be space, on their flight 1 hour later to make the connection.

Most likely cause for a delay out of SFO early morning would be fog, in which case you're likely no better off on UA metal then AC, especially if it is scheduled to leave later. However, since your ticket is UA-issued, I would assume they would also be able to put you on their non-stop SFO-PVG, if you are in danger of missing the connection.