Success! In case anyone is wondering how this worked out, here is what happened:
- My original itinerary was EWR-HKG-HND-NRT-EWR. United flights on the end. Several days in HKG with an ANA flight from HKG to HND.
- I don't have any status and I was in regular economy or premium plus for all legs.
- Unfortunately, I needed to drop HKG from the itinerary. If it had just been me traveling I'd have hung in there.
- I changed to EWR-NRT-EWR from the website successfully. It prompted confirmation of the itinerary before going through the seat selection and payment flow. The design of the flow could be changed to give customers a much better feeling of safety.
- I was initially presented with a view of the current legs. This was a bit confusing to be honest. Was it safe to delete the middle part from HKG to HND first? Should I start at the top of the itinerary? I decided to select the advanced option which was buried inside the top node which dropped me on the more familiar multi-destination flight search page.
- From the multi-destination search page I felt more comfortable deleting the middle bit, changing the first flight, and leaving the last part intact.
- This search ran successfully and gave me options for the outbound leg as if I was booking the original flight. I could see seat availability and pricing.
- Next screen gave me the return leg. I selected the same flight and class for NRT-EWR.
- Finally, it gave me the all up cost to change the flight and all the details in a view familiar to anyone who's booked on the United side before.
- Completing the change was straightforward like the original booking.
- In the process of booking, I backed out completely a couple of times to see if anything had changed. I was able to go back in and my original itinerary was intact. I'm guessing there is some risk if my beginning/end flights had not been UA as @WineCountryUA pointed out above. Losing the ANA flight (or experiencing some other issue) would have been annoying. but I felt the convenience was worth the risk, especially when I could browse flight options easily.