For all we know, Starfleet HQ had looked like that for the past twenty years or so, and it just so happened that the hero ship in TOS was ten years behind the times, and the hero ship in TMP was built (and decorated) to be ten years ahead of them.
I doubt that; during the TOS, it would be safe to say the "connies" were each at different points of their 5-year missions (no one ever said they were all launched at the same time), so ships could return to the shipyards, or spacedock whenever necessary for technological improvements. We do not know how much in-series time passed between "Where No Man Has Gone Before"
and "The Corbomite Manuver,"
but the ship did recieve many exterior and interior changes after that 2nd pilot. Then, the Enterprise had its engineering section remodeled after the events of season 1--no small undertaking--so its not as though the ship was a floating dinosaur so far out that the rest of of Starfleet tech passed it by to reach TMP levels while the Enterprise was away.
I think Enterprise was always recieving updates as required (or demanded), but by the end of Kirk's 5-year mission, Starfleet was getting around to changes to the "connie's" entire structure--which would not be addressed in a mere two years.
On the other hand, "hard" onscreen facts include Voyager 6 being more than three centuries old. That pushes TMP to 2278 at least, considering Voyagers 1 and 2 date back to 1977, and forces an eight-year gap between TOS and the movie.
There you have it. ST has always married itself to real world events, so your information serves as a fixed period of time between TV and film, and 8 years is certainly a more believable amount of time to justify the lead character's physical changes than what would be a shocking two years after TOS.