I don't think it's quite a simple as that, if we exclude the Stansted Express which is like for like replacement the remaining EMUs look something like this;
Existing EMU cars 644 = 161 4 cars = 53.66 12 car trains
New EMU cars = 665 = 133 5 car (equiv) = 66.5 10 car trains
On the assumption (and yes we need to wait for the details of how the new trains will be configured) that a 10 car trains will be the equivilent to an existing 12 car train, there should be around 12 additional long trains for the franchise to operate.
Secondly with just one fleet of trains (of 5 and 10 car formation) of a modern design availablity should be better than the disperate mix of 317s, 321s and 360s. Even if availability is only 2.5% better than today an additional 3 units should be available in the peaks.
My view is that the 10 cars could be used on Southend, Braintree and Hertford East services as PIXC busters, all standard class high density, whereas the 5 cars would have first and a slightly better level of comfort for the longer distance services but that's just my opinion.
Lastly if the new depot (at Manningtree or should that be Brantham really) is for the Stadler trains, the bi-modes could take over some of the quieter EMU services such as the Harwich branch and Colchester - Walton locals if this is the case another 4 x 5 car Aventras would be freed up for London services. This would also avoid any platform lenghtening at the smaller stations only served by the local services.
All this is speculation and time will tell what actually happens, but it's going to be an interesting few years come what may.
calling at one intermediate station at the most !
