Perhaps they'll use some of them initially for training and to supplement the 323s, of which there are barely enough to cover all the diagrams on Northern's current electric routes.
The EMU types concerned (317 and 319) are pretty much unknown in the north west.
Northern will need plenty of time for depot and crew training.
No doubt NR will want to run clearance/performance tests as well.
I think the last use of a similar type (322) in the area was the disastrous Man Airport-Euston service around 1999.
It's not at all clear if the new EMUs will interwork with the 323s (which are based at Longsight), or be kept separate (based at Allerton).
On top of which Northern's franchise extension is not yet finalised.
Plus the refurb possibilities.
Then NR/BB have to finish the Phase 2 wiring more punctually than Phase 1.