Move all the first pax to the boom aircraft and do all economy/premium economy in the 777 and you’d come out even in the number of seat miles available.

You could also upgrade a couple of the 777-200 or 787 routes to a 777-300 to get more seat miles to offset those lost to loosing a slot to a smaller aircraft.

Very true; I think the Overture is meant to be 50-55 seats? If so that's basically just the First/Business cabin of an American 777-300. If each of those 55 seats are paying a premium on top of sub-sonic business class, maybe that's enough to "lose" the remaining ~240 seats.

Edit: Apparently Overture is 65-88 seats depending on configuration.