It's an excellent question. Here's where we are:
Current (various) TOC plans mean at present, there aren't enough 170s available / will be available soon.
I haven't seen any
confirmation the "new" trains will be 170s, but it seems likely as there aren't any other units in a good number that would provide an upgrade to the 15x class (not enough 175s, 180s, 185s to fulfil the single stock type requirement for EMR Regional trains).
Abellio have said they will end up with just 3 types of stock, but I can't find any documents to link to this.
They would have had some sort of half baked plan on how this would happen before announcing this.
HERE is a potential plan to enable enough 170s for East Mids, but requires a lot of swapping about. However, all between Abellio companies so may be easier to accomplish and enable better fleet harmonisation.
Seen some people suggesting Southern will surrender their 171s.
But I haven't seen anything suggesting what Southern will get to replace them, whether they can logistically send their 171s to the Midlands, nor that Southern are looking to get rid of them. Red herring?
VOYAGERS CANNOT RUN TO STANSTED so XC
cannot give their 170s up.