Yes. You'll have to exit and reclear*.

While the shuttle that runs between UA domestic and international actually has a stop by the US Air gates in the same terminal as AS, it goes to the UA wing of the Intl terminal. BA is in the other wing.

* Unless you have a really long layover at SFO. Some AS morning flights that originate in SEA and PDX arrive at the Intl terminal because they continue on to Mexico. But the afternoon flights all arrive at the domestic terminal.