"John Rowland" wrote

Won't the first trains to use the new line be coming in rather than going
out? Or will they travel ECS from Waterloo to St Pancras?

AIUI E* are transferring from North Pole to Stratford depot at the same time
as they transfer from Waterloo to St Pancras. So I'd expect that trains
coming out of service at Waterloo on the last day there to run ecs towards
North Pole as usual, but to continue round the NLL and HS1 to Stratford.
Trains already at North Pole would be transferred to Stratford, and trains
coming in to service at St Pancras will arrive ecs from Stratford.

Peter