As evaluater says... We all hope that in due course the long term fleet (whatever size that turns out to be post the Afghan withdrawal and SDSR 2015) will be to the Mk6 standard as being purchased today. That said there are always some important and often hidden differences resulting from significant production date differences which can be quite hard to standardise on at a sensible cost. A long lifed production line like that of the CH47 is subject to changed construction standards (often to improve reliability and safety) and newer bits of kit and components, often as a result of design upgrades, or simply the need to change component specs due to old variants having ceased production.
Lets keep our fingers crossed and hope the cash is available in due course to get them all up to the same standard...everyone in the loop is well aware of the advantages of such a move, but if the cash isn't there up front, the best plans can fail very quickly.