Northern are always short of 323s - from memory there's 17 in the fleet, and 15 units are required.
The morning peak has at least 2 return services diagrammed as 2 x 142s - the 0606 Crewe via Stockport, returns as the 0739 Crewe to Salford Crescent, and the 0715 Macclesfield to Manchester. The 0715 runs up as an ECS working. Both of these workings are timed for DMUs - the Macclesfield timings are particularly slack from memory. I'd like to think that a 319 on these services would increase capacity and keep to time!
There's a couple of Hazel Groves that are booked DMU work from Manchester Piccadilly in the evening but nothing to Crewe/Stoke.
The other problem with the 319s is the length of the units. The Glossop/Hadfield line will require platform lengthening at Flowery Field and Godley at the least. They will only just fit a 323 and had to be lengthened for them when they where introduced. Other problems include the signal sighting at Ashburys on the Up, and how much of the train the conductor can see on the curve at Diniting Station. Not insurmountable problems but things that need to be considered.
All 323 work with the exception of Crewe is quite tightly timed. Anything with less acceleration/brake force will struggle to keep the timings. There's not much slack for easing them either - Glossop/Hadfield triangle is hard to keep time on as it is, on the Stoke run there's a voyager booked into Stoke 3 minutes after the 323 arrives and the airport run is busy at the best of times.
The 323s are well loaded at peak times, a few are full and standing. Recently some have been full and standing recently on some Christmas Market days - shorter units will not be helpful!
No-one at the depot has mentioned Piccadilly men signing the 319s. I suspect that they'll be staying over on the Liverpool/Wigan/Victoria side until the new franchise starts.