I'd guess the plan to reform some sets into 3-car units is probably one reason why replacing internal BSIs with bars hasn't been done yet. When the fleet plan was first announced there was mention on here of the 'boxed-in' /2 cabs being decommissioned and even removed, though that may have just been wibble. Has it been decided or made public which units will become 3-car yet?
According to the franchise agreement it's April 2019 before they are expected to enter service..