Because of the frequency of SFO-LAX flights, it's fairly easy to same day change. I know that same day changes are not allowed on international flights, but I think there is a good chance that if you got to SFO in time to get on the flight that leaves earlier that they will let you on. I don't think you will have luck calling in advance to make this change, but at the airport they are more likely to let you do it. If the Sky Priority check in agent does not agree to it, ask again in the Sky Club (not downstairs at check in, but upstairs).