Well, it depends on who you ask. The book Starfleet Dynamics posits that the Klingon philosophy is to just continually upgrade their ships ad infinitum until they are destroyed, also claiming that some Klingon space-frames have been in service since Earth's late 19th Century.
Personally, I love this idea, and it's backed up on screen, at least circumstantially, as we have seen D-7/K'Tinga type ships in Enterprise and Voyager, and all stops between.
Actually, I assume that Klingon ships are built and maintained by individual Houses rather than a central Federal/Imperial authority. This would allow many variations on the basic D-7 cruiser configuration, including the smoothies from TOS, the beefier metallic looking ones from TMP (and reused in TWOK and TNG), the luxury presidential one (TUC), the more brown and gray ones (DS9) and the CGI one (VGR, and ENT). (I'm sure someone will correct me if I got my DS9, ENT, and VGR ones confused, I'm not rightly sure off the top of my head whether the DS9 ones were CG or models, and if CG if they are the same mesh as used later or not.)
The point is, there are a bunch of "same but different" varieties of the D-7 type which could suggest refits or new hulls. Or variations on the same theme (my personal favorite notion).
But keep in mind: comparing the proportions of the miniature Richard Datin built for TOS compared to the model of the Enterprise refit that Magicam built shows pretty different proportions, enough so that to believe it's the same ship, just remodeled is quite a stretch. So the simple fact that the filming model of the TOS D-7 that AMT built in the 60's is differently proportioned from Magicam's K'Tinga shouldn't by itself tell us that they too are not just upgraded ships.
--Alex