If memory serves I recently read of a traveller who was in Europe recently on a RTW with a routing through Israel and they wanted to change/drop the Israel segment and pick up the following segment from another European airport. ANA wasn’t letting them modify the existing itinerary but the agent did let them know they could cancel in the middle of the journey and get a prorated refund for the segments not flown. So for example if you paid 100K for 10 segments and flew 2, in theory minus any ticketing change fees they charge you’d get 80K back. Obviously this isn’t something you’d want, but that’s an option according to a recent scenario I read off here. You could then maybe rebook close in if the inventory remains open after cancelling but that’s very risky.

Another option you have is letting the ticket go to “Airport control.” Once the checkin window opens for the flight, and on the day of travel, the Operating Carrier’s staff at the airport gain control of the ticket to make changes. In this case, since the Operating Carrier/Ticketing Carrier is the same (NH) perhaps the agents at the airport would be more willing/flexible to help on the spot? It’s risky to leave it to chance though on the day of travel; they may not be able to help or reschedule, and if you don’t fly the segment the rest of the segments will be cancelled.

Another option you could try is calling back to back NH’s call center but time it between the shift changes from US to Japan. You may have luck with a US agent but if not can hang up and try again with a Japanese one.