I am double booked on two different airlines (BA & UA) right now, for SFO-LHR-SFO. I got the upgrades on BA (much easier because they use the A and D inventory), but I am on the waitlist for UA. I would like to take UA, because the ticket is half the price, however, I don't really want to take it if I am not upgraded.

So what happens if I check into the UA flight (no bags to check), and I don't get upgraded. Can I get a refund on the UA ticket (assuming it is a refundable ticket) even if I checked-in for the first segment of the ticket?

The BA flight leaves a little later, so I have time to catch the BA flight if UA doesn't work out.