It was 7 years ago now but I used to fly ANA regularly to Japan with UA status, and 90% of the time they gave me a free upgrade to Premium Economy (which is a LOT nicer than Delta's C+). It wasn't the day of travel either, after booking I was able to select Premium Economy on the ANA website. Not sure if this is still the case, maybe ask in the UA forum.
Delta's C+ is a good product and if the seat map shows availability, that is more of a sure bet. But ANA may be better if you are more of a gambler.