It depends why the 757’s are on the schedule to begin with. We don’t know for certain, but I prefer to opt for the simplest explanation. It allows them to offer 2X frequency daily at a low operating cost. CDG bound you get to offer an earlier flight for people who want/need to be in Paris first thing in the morning, and you get to offer a flight to people who need to work a full day state-side and can afford to arrive in Paris mid-morning.
Maybe maybe not, it depends on what their intentions are for the route. This goes back to one of your posts that went unanswered in the diversion thread. Again, personally I choose to go for the simple answer.
Without knowing what their intention is (and to our United IAD friends I doubt it is Smisek carrying out his master plan to screw you over), it appears they want to offer an early and a late flight. I doubt they upgrade to two 76X. If they felt they could sell the seats particularly in front, they’d be operating bigger birds on the route right now. As it stands, it is likely cheaper expense wise to operate a 757 on the route even if you have to make a fuel stop than upgrade the flight (clearly if you could generate more revenue by selling more seats particularly upfront on a bigger plane the equation may change). Potentially reducing frequency again depends on their United’s intentions. If they want 2 daily flights, they obviously aren’t likely to reduce frequency.