SFO & OAK are co-terminals. However, Reservations will definitely (and always) charge you if you change a restricted ticket in advance. Also, for the person who asked about LAX-OAK, keep in mind there is less mainline service and mostly CRJs fly that route.

At the gate you can do it on a standby basis for free. But some agents know about the co-terminal thing better than others. More often, agents that work in "co-terminal" aiprorts themselves are more likely to remember and allow you to use this rule.

I've found good luck at LGA, LAX, and SFO this way. OAK, SNA, BUR, ONT less so but still somewhat better than airports that don't have a co-terminal.

I had a nightmare taxi ride from Manhattan to JFK. The driver finally admitted he'd never been to JFK in his life, and since I didn't know how to drive there when I saw the sign for LGA I had him drop me off there. I had completely missed my JFK departure but when I went to United they were happy to take care of me by switching me from JFK-LAX-SFO to LGA-DEN-OAK (standby for free, or $100 plus fare difference if any for a confirmed space). They also told me quite clearly how well each segment was sold. Cleared both and DEN already had my boarding pass on the counter waiting for me before I can even ask them if I was still on their standby list. Really made me feel good about United.

Please be careful, though. As I'm sure you know, certain standby situations feel more like Press Your Luck with Peter Tamarkan. If you hit a Whammy at the end of the day, your wallet could get trashed. Good luck!