A slight exaggeration. What Northern's website actually states for the North West is:
Northern | Buy Cheap Train Tickets and Check Train Times
www.northernrailway.co.uk
Nevertheless, currently Northern has twelve 319s diagrammed in traffic M-F, of which four diagrams are peak only. There are no cuts to the timetable, so if all the 319s are withdrawn those 12 must be replaced by other stock.
Currently one 323 and one 331/0 are diagrammed as stabled all day, so if they are brought into traffic, e.g. on Liverpool - Wigan diagrams, that leaves ten 319 diagrams to be replaced.
Reported upthread that the NW is gaining 12 331/1s in exchange for 10 331/0s. Currently two 319s are diagrammed on Liverpool - Blackpool services, which the website implies will remain 4-car, so that accounts for two of the twelve 331/1s. In order that "some" of the Liverpool to Wigan services remain 4-car as Northern has promised, at least one 331/1 will have to be diagrammed on this route to replace a 319, leaving seven 319 diagrams to be replaced. Also a /1 can take over a /0 Liverpool to Airport diagram in order that "some" of those services become 4-car as promised. The /0 would then take over another 319 Liverpool - Wigan diagram, leaving six 319 diagrams to be replaced. Assuming at least 10 of the 331/1s in traffic, six 331/1 diagrams could then replace double 331/0 formations on the Airport - Blackpool route, releasing another 12 single 331/0 diagrams. Currently a total of 18 331/0s are diagrammed to work in 9 pairs on Blackpool North services, so there would still be "some" 6-car services as promised.
Currently three 331s are on exams at Allerton daily. Assuming this remains the same with the slightly larger fleet (the mix of /0s and /1s would vary from day to day), still only four 331 diagrams (including up to two /1s) would be available to replace the remaining six 319 diagrams not accounted for above, after sending the ten 331/0s to Yorkshire.
How could the deficit of two diagrams be covered? One 323 diagram is currently allocated to crew training. I do not know if this will be ongoing after December. Also one 323 and one 331/0 are diagrammed to be away at Wolverton on heavy maintenance. I do not know if these programmes will be ongoing continuously. Otherwise DMUs might have to work a diagram or two, some weeks at least.