Chances are you won't be able to do the 500 miles - if its the same flight number, you'll be credited with direct SAN-ORD miles.
For the second, check at check in (use OLCI...or at points thereafter), and as a gold, you can do a same day change for free, assuming that the fare class you're booked in is open. You can either change to an earlier SAN-SFO flight, or they should, again, if its available in your fare class, offer you a nonstop option.
Note that the options offered will be within 24 hours at the time you are looking, so if theres a nonstop departing an hour after your original flight that you are looking at, you won't be able to see that until T-24 for that flight.