Perhaps 6 car 323s elsewhere will allow 4 car 331s to be shuffled onto the route
They say it's going to happen but it's not. There are very few routes where doubled up 6 coach 323s would actually work - they would have to optimise certain routes to only call at stations which can accommodate 6 platforms and skip stop on platforms that were too short as they don't have selective door operation.
Blackpool North to Manchester Airport is one potential route but do recall someone on here saying Network Rail don't like 323s operating through Bolton as something on them appears to interfere with lineside equipment. Not to mention the spacious 195/331s are well suited to Manchester Airport routes (in terms of luggage not blocking doors) so not sure what is gained by replacing them with commuter style seating.
The 17 323s are going to be a like for like replacement for the ~15 319s in service. The real reason Northern have taken the 323s is Porterbrook realised Northern are the only possible takers and wanted the 319s back to potentially convert them to 769s. The remaining 319s will be written down and send them for scrap or spare parts for the remaining 769s. This strangely has the effect of increasing leasing costs as there is an oversupply of EMUs at the moment.
For Northern, the big benefit is it's one less mini-fleet in the 15 or so 319s to deal with so less training and maintenance costs. The "random unit generator" has been a millstone around the Northern franchise neck for years now and one of the reasons subsidy levels are so high as you're having to train a lot of staff on a lot of different units for zero revenue. In terms of 769s these are a stop gap and I would be surprised if bi-modes 331s are not procured in the coming few years to replace them.