This is a little different to your experience, but a couple years ago (when their res office was still in Honolulu) my HA flt was gonna be delayed for 16 hrs. Because I didn't want to be stuck at LAX for 16 hrs I wanted to change my inbound connecting flight (booked as separate reservation). The agent kindly called US Airways and they changed my flight w/ no fee. According to the HA agent, "airlines help other airlines" so I would recommend you ask the HA agent to call United and see if they will change your res for you w/ no fee.