TfW still have 11 170's that are eventually due to go to EMR. They were to be replaced by the new Stadler Class 231, but TfW have now taken the decision to introduce the Class 231s on the Rhymney line to cover the dire performance of 769s, until the Class 756s arrive for the Rhymney line. That will likely mean a delay to 170's leaving TfW as they really can't afford to lose more of the fleet.
I read that (It was on Wiki though!) the introduction of more 170's from various TOC's will see the 153's (Already retired), 156's and 158's leave EMR.
Is there a plan on where the EMR 158's will go yet?
As TfW have 11 170's but only use a max of 8 per day, it could make sense to swap the 170's for 11 158s. EMR will benefit from capacity increases as there are 8x 3 car and 2x 2 car 170's from TfW. TfW will benefit by not having to maintain and train a small sub fleet. They already have 158's in the fleet, so they will be route cleared for many more routes than the 170's are currently cleared on, and Canton Depot can maintain the EMRs 158's as they won't have ERTM's. For routes where 3 cars are required at peak times, TfW have quite a few PRM and PRM lite 153's that can be attached to 158s.