I'm not aware that there is an intention to withdraw any Class 15x vehicles from service once the Class 175s are fully introduced. Whether the Class 185s would be suitable to replace two car Class 150s or Class 158s is a fair question, through.
Personally I'd convert the remaining two car class 150's to four car hybrids (in 150/1 [52], 150/2 [57], 150/2 [52], 150/1 [57] formation), with the remaining two car class 158's being converted into three car hybrids and used this way until they're ready to go in the bin / be replaced / be stripped of spare parts to keep rest of said fleet going / be sold to some foreign nation somewhere.
The class 150's are simply used as infill units (rural branches) or as hot spares incase a class 185 or class 195 is out of action or even for the Friday/Saturday night vomit comet express.
This would allow the areas which currently use the 158's to lengthen them. As awful as it is, it'd make sense to send a majority of class 158's to the East Midlands, South Western and Great Western due to staff familarisation and retaining a small, but reasonable number, to keep the services in the North East going.
As for the class 185 "Pennines", stick them on the Leeds - Wigan (which should be extended back to Southport), Leeds to Carlisle via Settle, Leeds to Nottingham, Sheffield to Scarborough and York - Blackpool services to free up said class 158's.