Really frustrating situation.

Had a ticket in regular Y going LAX-SFO-BOS for a value of $278.

Changed it to a regular Y $99 LAX-ORD flight. The final change screen showed that I would receive a residual value of $178.

However, never got the residual value after the change, and can't find it under my MP account.

Called in UA, and the agent/supervisor tells me I won't get any residual value. Nothing they can do.

What should I do next?

You booked directly through UA?

Yes, all through UA.

In that case, I'd do the following:

1 - Scour your credit card statement to see if you actually got a refund. (This has happened to me).
2 - I assume you've done this, but just in case: search for the residual value in your account using your confirmation number.
3 - If neither of those help, file a DOT complaint: https://www.transportation.gov/airco...umer-complaint That'll get a more experienced UA agent to look at it, and will likely lead to a positive outcome.

Clarifying on this point: most direct way to search is to use the original confirmation (the one for LAX-SFO-BOS) and your last name to try pulling up the residual at https://www.united.com/en/US/fly/travel/credit.html; if the original confirmation doesn't work, try the new one (LAX-ORD). Sometimes it won't show in your account, but it will be active as a credit.

When I did the change online, the LAX-BOS itinerary was still active, meaning I never canceled it. I just made a change to the cheaper LAX-ORD flight. The PNR is the same for both reservations.

According to the supervisor the LAX-BOS fare rules don't allow any residual value if changed to a cheaper fare. However, on the final page before confirming the change, it showed I would get a a new credit of $178, and basically all non BE fares allow flexible changes with residual values being given back.

Not sure what is going on.

Have you tried searching for the credit? Weirdly, they can have an active reservation and residual value with the same confirmation.

If you've searched and not found it, DOT is the best route. The fare rules are often written with a no-residual clause, but it's been overridden by a waiver for a couple years now.

inThis. That's why I asked if the ticket was purchased from UA. UA has apparently been telling travel agents to enforce this rule, but has been waiving it for purchases made with them directly. They've left some wiggle room in their policy -- "If your new flight costs less, you may receive a future flight credit for the fare difference if the fare rules allow" -- but (a) they've been consistent in their application of allowing residual credit for all non-BE flights booked through UA, and (b) they explicitly told you that you'd be eligible for residual credit, so this is an absolute no-brainer.

In the future, I recommend cancelling out a reservation and then using the resulting FFC, rather than doing a change. It does have a minor downside -- if you run into a situation later where you're due a refund, the refund becomes FFC -- but, IMO, the upside of having a clean PNR is worth it.

You need someone to look through the e-Ticket history section of the PNR. They should be able to see when the ticket was exchanged and where the value for the fare difference went as part of the exchange. It should indicate if it was refunded to a credit or to the original form of payment. If there isn't, the value is stuck in limbo and probably needs to be teased out by the refunds department. As others have indicated, you may have better luck with the DOT as it will ensure getting the right people to look at it as opposed to phone lottery if you get lucky to find an agent who knows how to review eTicket history.