You have both standby and 'same day change' (SDC, look up a separate thread on that) as options.
It's fairly common and shouldn't be a problem. Do you have status on UA (or *G)? SDC fee is $75 (waived for UA's Gold and above, possibly *G too), standby at connecting airport is free.
DO NOT recheck your bags at SFO before seeing an agent, luggage will (in most cases) block SDC/SBY.