163627,
In simple terms, there will always be at least two fleets - the thin tank (Mk4/6) and fat tank (Mk3/5). In an ideal world the MoD will upgrade all of the Mk4s and Mk5s to a Mk6 (Mk7?) standard by fitting the CH47F (Mk6) unique "bits" to the Mk4/5s. There's already a lot in common - most of the differences revolve around the fuselage construction and Digital vs Analogue flight control systems. Interestingly, when faced with the rebuild from D to F, in many cases the US Army simply junked the fuselage and built new ones - it wasn't that much more expensive in the long term by the time the old frames were cleaned and repaired.
Any commonality programme will depend on how the finances go and how busy the fleet is post 2015. IMHO, the best thing we could do is order another 50 Mk6 ac in a variety of thin/fat tank configurations and sell our old cabs on the open market (except for -718, which really must go to Hendon..). Mind you, they have been "well used"...Hope that helps!!