The 68s should be in use until at least after the Commonwealth Games (which run until Monday 8th August) as Chiltern will need the capacity.
After that, who knows? Ultimately they'd need twenty-four 100mph capable D(E)MU vehicles (preferably Turbostars, for compatibility with the existing fleet) to replace them, and those are in fairly short supply.
I think that hankering for more turbostars is wishful thinking, they're already in short supply in other parts of the network,and those that are around nearly all have pending transfer loactions, dependent on acceptance of the new WMT and TFW stock.
I would have thought that the 68's will be held onto until some of the other operators such as avanti WC and EMR start getting their new toys,but optimistically,in order to release the voyagers/meridians/adalante's you'd be looking at at least 12-18 months from now before those are type approved and crew trained.
You could pehaps have a bit more luck getting hold of the tfw cl175's.There's 27 of those,but they are split 50:50 between 2 and 3 car formations, so could encounter some capacity constraints.
I think if I was playing trainsets with chiltern, I would probably be considering the class 180's(all 14) for the birmingham-london expresses, and the class 175's to replace the clubmans and turbostars.All the Alstom stuff can be maintained in birmingham by their OEM,which is not far away.