I believe that when the Northumberland line opens, the plan is for 158s to be used exclusively on Tyne Valley, Morpeth and Northumberland line services, with other Heaton-based North East diagrams using 156s. 3 extra 158s have already moved to Heaton to allow for this. This ties in with the continually pushed back ECML retimetabling, which will see all Northern services to and from Newcastle being self-contained i.e starting/terminating at Central Station.