Perhaps; perhaps not. Partially-used tickets are weird. In most cases, this will represent an upper bound of the price to be charged for HNL-EWR, but it depends upon the outbound fare rules as well. (Of course, OP always has the option to cancel the return trip entirely, eat the cost, and then pay for HNL-EWR separately).

Except the credit from OGG-HNL-EWR is likely to carry along these restrictions. If the outbound flight was a round-trip fare — likely, but not guaranteed, for Hawaii travel — then the generated flight credit could only be used for a flight from the islands. And using the FFC to book HNL-EWR should get you the same price as a simple change would — although I admit, I can’t figure out why UA is being so challenging about making this change.

OP: Don’t mention previous calls or try to explain. Just say “I’d like to change my return to depart HNL instead of OGG.” I suspect part of the problem is that you’re over-explaining.