I see two likely reasons for increase coach, decreasing first.

1.) They figure they will get more revenue by selling more seats since a majority of fares in the front cabin are Y that have been upgraded. Gives you more Y seats to sell. If they were really selling those seats at true F fares, I think the choice may have been different.

2.) Fleet simplfication as part of the integration. If there is an equipment swap for the same aircraft type, it makes operations at the airport much simpler. Less downgrading, upgrading etc. CO has tons of problems with this since they have many versions of the 737 they swap frequently.