United Airlines - possibly changing flight time

I am after a bit of an opinion and advice here and hope someone can help. I'm flying out to the States on VS007 LAX at the end of this month and flying back on VS020 SFO in mid-November. I have a connecting flight when returning from San Diego to San Francisco on United that gets into SFO at 3.15pm (flight change of time just received). VS020 leaves at 4.45pm. Aside from the fact that it won't give me much time in the well lauded Clubhouse at SFO, do you think this seems a bit rushed? I will need to get from Terminal 2 to Terminal 1 at SFO and am thinking that any delay on the domestic will leave me a bit tight on time. Can I check my bags in at SAN straight through to LHR or will I need to check in again at SFO (I am assuming I do)?
The second part of my question is, having been told by my TA that I can't change the United flight until I have started travel, how easily do you think I will be able to get the earlier flight which gets into SFO an hour and a half earlier and which makes me more comfortable? I'm going to take a printed copy of their website 1-800 numbers for reference.
Thanks for any advice you can give this connecting flights novice[:I].
Phil
The second part of my question is, having been told by my TA that I can't change the United flight until I have started travel, how easily do you think I will be able to get the earlier flight which gets into SFO an hour and a half earlier and which makes me more comfortable? I'm going to take a printed copy of their website 1-800 numbers for reference.
Thanks for any advice you can give this connecting flights novice[:I].
Phil