ORD-NRT to the day earlier. No change to NRT-SIN. All segments, including the last one, were listed on the rebooked ticket in the same (W) booking class.
None of that made any difference to UA CS. According to UA, NH no longer has this fare available on the original flight, and there is nothing they could do about it.
UA initially offered to either upfare for $$$ or refund my ticket.
To close the loop, I'll say that to avoid fighting UA on this I agreed that they rebook NRT-SIN a day earlier (there were W fares available before and after the date I was booked on, just not on that date). I suppose in theory I could have fought this all the way to DOT, but that is not an effort I want to make.
UA IT strikes again, I guess.