In SFO you have to leave security and make a 10/15-min. walk on foot or go up a few levels and catch the Airtrain from AA Terminal 3 to AS Terminal 1.

Map here:
http://www.flysfo.com/web/page/atsfo/airtrain/map/

With the airport as a clock, AA is at '10:30' and AS is at '3:00'. Like all airports, security lines vary greatly depending on the time of travel.