I'm not sure I totally understand your situation. Why do you want to keep the IAH-BDL trip? Your employer will hopefully be understanding enough to still reimburse you for any expenses you incurred for a trip you can no longer make. Or, you might consider asking your employer to pay a change fee to reschedule your BDL trip to a later date.

Assuming that your employed will reimburse you regardless and you no longer need the BDL trip, I would recommend calling UA, explaining the situation, and asking them to change your itinerary to IAH-SFO so that you may apply the fare you paid for the BDL trip to your SFO trip. When they explain that you will incur the change fee, ask politely if they will waive it as a courtesy. If they decline (which is likely), ask them to cancel the IAH-BDL trip. And then go ahead and book your new flight.