If you want to play "fantasy swaps" then what about the Northern 170s going to EMR to directly replace the 158s?
Northern don't really need the 170s (as they've got 195s for the longer distance services - e.g. Bradford to Nottingham), a bigger fleet of common 158s may be a lot more useful to them.
Otherwise, I'm not sure that Northern are going to need the EMR 158s - AIUI there's requirement in the Northern franchise for a number of modern DMUs (I think it's worded as being "similar"/"equivalent" to a 185?) but 158s wouldn't fit that requirement, so the prospect of getting EMR units would have to be on top of the units mentioned in the franchise requirements.
(that's assuming that we don't get a "DOR" version of Northern which rips up the requirements of the current franchise, of course)