Its really just 1 of those we must wait and see items.

I cant see how the merged Carrier will beable to have say a 777 flying EWR-FRA with 2 cabins and IAD-FRA with 3

either CO will have to roll out E+ or they will need to remove it from UA as well as F/C/Y with UA and C/Y with CO.

too many people will stop flying UC if they paid for F only to be downgraded to C since there was an equipment swamp from an old UA to a CO plane. Or a A319 is being subed for a 739 and now only 8 FC seats

it might be possible say to keep 3 cabins TPAC and using only UA planes for All TPAC flights and using 2 cabins TATL for ALL flights but even that can mean problems

But there are numerous issues that will have to ironed out , could be they have been already but they just are withholding announcing them outright for now