Just to be clear, inventory refers to fare class inventory availability. If you are just looking at seatmaps, that's not the same thing. The cheapest fares to HNL have a roundtrip booking requirement with a Saturday stay requirement. If you attempt to make a change and no longer meet the min stay requirements for roundtrip fares, they will upfare you to more pricey one-way fares. There are also separate fare filings to HNL for Mon-Thu travel vs. Fri-Sun travel (which are charged more). Advance purchase requirements also play a signficant factor here and close-in bookings or changes will no longer qualify for the cheaper advance purchase fares. There are various fare filings with 30, 21, 14, and 7 day advance purchase requirements with the cheapest fares requiring the longer advance purchase. Potentially, they might be able to use original booking date to measure advance purchase for fare on new return flight after outbound has been flown. But fare class availability, day of travel, min stay requirements for roundtrip fares, etc. are all still factors in these cases. While most fares are priced on an O&D basis, UA and AA domestic fare rules are pretty liberal about fares allowing combinations on a segment-by-segment basis with each segment on a separate fare in cases where this may be cheaper than O&D fares (DL, not so much). Most domestic fares don't allow stopovers and any layover greater than 4 hours will create a fare break (causing each segment to be priced on a separate fare). Hawaii fares are an exception and allow for stopovers in ORD/DEN/LAX/SFO/IAH/EWR/IAD for an addtional $60 fee.