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.