Bumping this thread because it seems to be the most closely relevant thread to my situation. I am due to fly on the following routing: HKG-HND//NRT-LAX-JFK where the stop in TYO is <24 hrs, but the onward happens the next calendar day.

I would like to take the flight HKG-HND, but then change the ticket to add a stopover at a later date. Effectively cancelling NRT-LAX-JFK before the NRT flight departs, and plan to change the dates at some point in the future when I have firmer plans.

The fare (HKG-NYC) permits stopovers en route at 100 USD.

However, 1K/Rate Desk is telling me that all changes have to be made before I fly HKG-HND, and I can't insert a stopover once I've flown HKG-HND, and I would lose the remaining value of the ticket if I didn't fly onward from TYO.

This last part seems incorrect to me. My experience with other airlines is that provided the original fare rules permitted, I could change dates en route and pay the applicable fare difference & change fee. Forfeiting the value seems incorrect provided I let them know in advance of the NRT-LAX-JFK flights that I wouldn't be flying.

Any UA experts that can chime in here?