They would rebook you for free if you missed it due to the delay (since you booked the two flights married together), but obviously since it is a long haul that is not ideal given that the next flight probably is a while later, and you might get stuck in a middle seat.

You cannot drop the first leg and keep the same exact fare; your fare is always contingent on you keeping the same origin and final destination, even if you are flying one less flight. What you can explore is cancelling the ticket (pay the award redeposit fee) and rebooking just straight out of SFO, but since you don't have status, that will cost you a bit. If you are lucky, UA can find a way to drop Leg 1 in a cheaper fashion, but technically you can't do it without sweetalking someone.

The same day flight change option mentioned above is also an option, but it would only allow you to change if the same fare bucket is available; if you booked Saver award, saver award would still need to be available the day of the flight and the itinerary with the earlier connection would also have to be a route sold on UA's website (just search for a new booking for that day and see if it exists and what fare buckets are available). In addition, you have to pay $75 per person as non-status, but that isn't too crazy.