Other than unrefurbished units coupled up to demonstrator units (eg 7008), do unrefurbed units often end up coupled to old ones. I would guess that if (like most refurbishments) there are adaptations to inter unit connections, for PIS-es and the like, they wouldn't.
Last thought, when 7008 was doing the rounds as the demonstrator, did it ever seem more busy in 17008 than in any other car, or even in the 'new' half of the train, especially off peak when there are more leisure travellers?
"I don't make mistakes. I make prophecies which immediately turn out to be wrong." "And now, excuse me while I interrupt myself" - Murray Walker OBE
Student of Mechanical Engineering at the University of Nottingham (loosed upon the real world for a year)
Those are the really obvious changes. The other ones are more subtle, eg plating over the external door open buttons, new flooring (that is flat - not bubbled!), new treadplates, plus some others. This thread should give you some more detail: