Why did they end up doing the refit for the movies? Did they strike the set right after the show and not expect to do the movies, so they had to build a new set, or was there a different reason?
The short, snarky answer: “The show was canceled in 1969. The movie didn’t start filming until 1978. Of course there were no sets around!”
The longer answer: There was talk of a lower-budget Star Trek feature as early as 1972, when D.C Fontana brought it up in an issue of Star-Borne magazine. Gene Roddenberry approached Paramount with an idea for a movie called Star Trek: Planet Of The Titans in 1973, which was abandoned in early production but featured a new Enterprise designed by Ralph McQuarrie and Ken Adams. (Shown below.)
The Enterprise refit was then going to be a key element of the plot of “In Thy Image,” the pilot for Star Trek: Phase II, a show that was going to be the flagship of a new Paramount network. Here’s Matt Jeffries, talking about what he did:
“Basically, what I did to it was change the power units, and make a slight change in the struts that supported them. I gave the main hull a taper, then I went flat-sided and thin with the power units, rather than keeping the cylindrical shape. Trying to work out the logic of the refit, I knew a lot of the equipment inside would change, but I didn’t see that there would be any need to change the exterior of the saucer. Certainly, though, the engines would be a primary thing to change. Part of the theory of the ship’s design in the first place was that we didn’t know what these powerful things were or how devastating it would be if anything went awry, so that’s why we kept them away from the crew. And that meant they could be easily changed if you had to replace one.”
It’s that Enterprise that’s featured in the original teaser ads for Star Trek: The Motion Picture, but as you can tell, there are a number of differences between that and the final movie version of the ship, which was redesigned by Andrew Probert under the aegis of art director Richard Taylor. A lot of this has to do with the fact that the Enterprise model for Phase II was designed for television, and they needed something that would look good projected onto an 80-foot screen.
Additionally, interior sets were in the process of construction when Star Trek: Phase II’s production went on hold. Featuring designs by Mike Minor, they’re obviously a half-way point between the original series and what we see in the movie, which featured the work of Taylor and the rest of the team from Robert Abel & Associates. With a larger budget, Taylor’s team was able to re-build and upgrade many of the sets. (Interestingly, the bridge and transporter room underwent the least amount of work.)
TLDR? The refit was part of the plot from the very earliest stages of production and became even more necessary when they moved from a second television series to an actual motion picture.