1. BA will not check bags through to UA, so you will need to purchase whatever baggage allowance they require for the UA flight.

2. Should be possible to change BAH-LHR-IAD to BAH-LHR-SFO. You will have to pay change fee plus fare difference. You'll need to call BA to get a quote for this, but if you post full details of your booking (original and new flight dates, fare paid and ideally fare basis, booking date) then one of the talented chaps here should be able to work it out for you.