Delta will give you a full refund since the schedule change is greater than 30 minutes; however, they won't book you on UA unless it's day of due to a mechanical delay or cancellation. You can't be rebooked on a mainland-Hawaii flight on HA because DL only partners with HA on inter-island flights, not flights from the West Coast to Hawaii.

Looking at the flight schedules, the only other "DL" option to KOA is DL from SFO to HNL departing SFO at 3:00 pm then connecting to an HA flight that arrives in KOA at 7:35 pm (well, for the day I looked - the schedule might varry slightly). Not much of a difference but if you want to stick with DL, it's the only "earlier" option.

EDITED TO ADD: Meant "Delta will give you a full refund since the schedule change is greater than 90 minutes", not "30 minutes".