USS Einstein wrote:
...the impression I have always had, was that they were seperate classes, or the K'Tinga was a refit of the D7, akin to the Constitution being a refit.
Generally, I think most look at it that way.
Whereas the Klingons gave the refit of the D7 a new designation, Starfleet opted not to do that for the upgrade of the Constitution-class. The naming of ship classes can be as arbitrary or politically motivated as the naming of individual vessels in that regard, IMO.