Would your company be willing to help you re-route IAH-SFO-BDL with a stopover? Basically, would they let you change the ticket and pay the difference, then expense the original amount? Same day change won't really help you as you would be changing to a completely different airport. Your best option if you want to fly IAH-SFO is to book on another airline. I did something similar last year - had a SCE-ORD-DEN flight booked far in advance, then needed to go to FRA for work, so I booked the ORD-FRA flight on LH and then just told the UA gate agent in ORD that I wasn't taking the ORD-DEN flight. IIRC, the SCE-ORD-DEN flight was an award flight.