If you rebook an award ticket through normal UA customer service channels, the rebooking (a) is subject to current award availability, and (b) will result in a reprice based on current award prices. The first agent should have explained all this too you before diving in and making changes.

It's theoretically possible you could get the original award flights reinstated at the original price plus the no-extra-charge addition of LIH-OGG, but that would require supervisor involvement, lots of time on the phone, and a fair bit of good luck.