They won't. The mk3 sets and the units currently being subleased to Northern are needed to allow the North TPE enhancements - the extra Manchester to Leeds service and extending all Airport-York services to Newcastle. All TPE Manchester-Sheffield services are expected to be 6 car once the new stock is in service.
Thats a shame but it makes sense with the Mark Vs not arriving until August and the 185 upgrade program running slightly behind schedule.
Will even six cars be enough for some services?
Intially yes but in the long term it depends on how much demand is being surpressed by overcrowding. I have suggested that 9 coaches may be necessary using some of the units going off lease at the end of 2019. However, it was pointed out that some of the platforms on South TP are too short and Manchester Airport platforms would be very expensive to extend. Perhaps reconfiguring the fleet into 4 and 2 coach sets with 1 first class + 3 standard and 2 standard only units, which would involve little more than seating and coach swaps. The platforms on the skip stop North TP service could fit doubled 2 coach sets but not doubled 3 coach sets and doubled 3/4 coach sets would fit South TP. The Rosco would have guaranteed a home for a larger proportion of 185s. That would mean a TPE 185 fleet of 8 x 4, 8 x 2 and 17 x 3. That would send 18 x 3 coach 185s off lease instead of 22.
If the Liverpool to Norwich service is split / merged with other services and Liverpool to Nottingham transferred to TPE then it would likely be run with 6 coach 185 sets which would be small increase (50-60 seats?) on doubled 158s.