An intriguing missing link exists in 'Star Trek: Phase II'. Now, while TMP's sets and overall design were originated in Phase II, I can't help wondering if the Phase II refit wouldn't have come across as a bit less of a visual leap than what we got in TMP. We already know from the test footage that the existing TMP sets were orginally painted in a more TOS-like color scheme, and the actors (in those screentests at least) wore variations of TOS uniforms. The Phase II writer's guide even goes as far as to suggest that the new Enterprise will look a little different, but still be cosmetically the same ship as the one in TOS.
My theory is that when Bob Wise came aboard he shook things up a bit (revamp the sets, bring in new uniforms). In so doing, the narrative device of it simply being a 'refit' kind of went out the window, because as characters in the movie itself note it was effectively a whole new Enterprise... they'd probably have been better off just running with the idea that the TOS ship was decomissioned after Kirk's 5 year mission and replaced with a brand new model. The differences between the old Enterprise and the movie model are too extensive for it to be anything other than a ground-up build IMO.