Sounds like a similar situation to the East Midlands Sprinters not being able to work with ex Anglia 156s. The PA/PIS systems won't work together and with the wonders of modern technology both would crash if it were tried.
As PIS is a PRM/DDA requirement, such an eventuality would render two units in need of repairs and unusable for a time. Therefore better to keep units with incompatible systems apart, to keep as many in service as possible.
Of course, in a sensible world, the units transferred in would be fitted with a compatible PIS, to avoid such problems. But if the hope is to have new trains in service to replace them before too long, perhaps it was deemed unnecessary.
Here's hoping I haven't just opened up a can of worms!