improved in capability over the years (CH47,
You didn't pick a very good example.
The pre-production Boeing Vertol YCH-1B made its initial hovering flight on 21 September 1961. In 1962 the HC-1B was redesignated the CH-47A
That's fifty two years and forty six days ago. The Hercules is even older.
A large number of helicopters have been improved over their lives to such a degree that they are unrecognisable to the original, e.g. Bell 47. A helicopter, like any thing else that flies, is a delivery system. Should the airframe still be suitable for purpose then there is no reason why it should not be improved by replacing obsolete engines or equipment. e.g. Boeing B52. The Puma started by changing the cockpit structure in the late sixties and improved through the Super Puma, Super Puma Mk2 and the 225. All have basically the same fuselage; larger windows, different backside but still the same top deck and lower structure.
For the RAF its a good deal. Their aircraft have peanuts in the way of hours. A multitude of commercial examples have in excess of 25,000 and even when they are retired from their original role they carry on with other operators. (North Sea to German police) and (Offshore Australia to US Navy vertrep.)
Even the old Sikorsky S61 has had a new beginning with a deal signed for used airframes in June 2010.
The U.S. State Department has entered into a five-year indefinite delivery, indefinite quantity (IDIQ) purchase agreement for up to 110 modernized S-61 aircraft for passenger and cargo transport missions in support of its worldwide operations.
Sikorsky website.
Two of them, one an ex North Sea veteran, and I mean veteran, are operating out of Akrotiri.