I tried CPH-NRT. I was rejected about 36 hours before flight. In the end, I think there was only a few empty seats in M. I upgraded to Y with points. 8 empty seats there. The gate agent told me that Japanese groups cause these flights to be very uncertain and that the flight was going out with only 20 empty seats.
I was the only M-Y upgrader because the seatmap was exactly the same as the last check on Expertflyer about 48 hours before. In M there was about 60 unoccupied seats according to EF. At online check-in window it was down to about 20.