I think you misread the post. For those flying in Y on MSP-NRT the introduction of the 777 will be a nice improvement over the 747s as currently configured.
I didn't misread it.
My point is there will be 104 fewer coach seats every day to NRT (and 20 fewer BE seats every day). That's a significant cut in available seats on a flight that is nearly always full.
For those 124 extra seats, they'll have to connect thru somewhere else, likely the West Coast, which is served by 763ERs with (you guessed it) a coach product very similar to the 744s. Or they'll be routed through ATL or DTW.
When fleet upgrades happen, they rotate the changes through, not make a schedule change for the entire season.