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In Star Trek The Motion Picture it is mentioned at least twice that the Enterprise has been undergoing a refit. Being that this was the first Star Trek movie following the TV series, the choice of word, refit, would seem to imply that this Enterprise is the same one as seen in the TV show with some repairs and replacements.

From the movie:

[Scott to Kirk, prior to the shuttle pod flight to the Enterprise]

Admiral, the Enterprise has just finished eighteen months redesigning and refitting. She needs testing, a shakedown...

[Alien Ensign to Sulu, on the bridge after Kirk's exit]

And Captain Decker? He's been with the ship every minute of her refitting...

However, the radical change in appearance would seem to belie the wording. Aside from the general shape, all the lines of the ship have been changed and all viewed interior rooms have also been radically altered. At the very least these changes would suggest that the ship had been upgraded, but more likely one would believe that this is in fact an entirely new ship named Enterprise.

Is the Enterprise in The Motion Picture actually intended to be the same Enterprise from the original TV series, with an awkward choice in wording that they called it a refit, or is it an entirely new ship named Enterprise?

re·fit/rēˈfit/
Verb:

Replace or repair machinery, equipment, and fittings in (a ship, building, etc.).

up·grade/ˈəpˌgrād/
Verb:

Raise (something) to a higher standard, in particular improve (equipment or machinery) by adding or replacing components.

Consider the USS Intrepid (Museum in NYC now, served from the end of WW2 to 1974, I was just there the other day) aircraft carrier.

During a refit in the 1950's they changed the deck, from a straight deck to angled, to handle jets vs prop fighters. This is pretty analogous to the change from round to sort of rectangular warp nacelles. The deck of an aircraft carrier is pretty distinctive and big, as are the nacelles. (Conceded there is not THAT much to the deck, vs the nacelles with the warp engines in them, but you get my point, not a perfect analogy).

They did lots of work on the interior as well, per the displays at the museum, so seems likely calling it a refit for such a major update is reasonable.

This is from Memory Alpha:

System upgrades with new technologies after long deployments were far from unusual in her history, but the Enterprise's overhaul of the early 2270s became a nearly keel-up redesign and reconstruction project.

The very heart of the ship was replaced with a radically different vertical warp core assembly, linked to new, and heavier, warp engine nacelles atop swept-back pylons and integrated with the impulse engines. The new drive system allowed for an expanded cargo hold in the secondary hull, linked to the shuttlebay.

Weapons system upgrades included nine dual-phaser banks with power channeled directly from the warp engines. A double photon torpedo/probe launcher was installed atop the secondary hull.

Multiple egress points now included a port-side spacedock hatch, dual ventral space walk bays, four dorsal service hatches, and a standardized docking ring port aft of the bridge on the primary hull; four more docking ring ports, paired on the port and starboard sides of the launcher and secondary hulls respectively, and service hatch airlocks on the port and starboard sides of the hangar bay's main clam-shell doors.

A new bridge module reflected the modern computer systems, operating interfaces, and ergonomics that ran throughout the ship.

Following Kirk's promotion to rear admiral and posting as Chief of Starfleet Operations, his hand-picked successor, Captain Willard Decker oversaw the refit, assisted by Chief engineer Commander Montgomery Scott.

After two-and-a-half years in spacedock for refit, the Enterprise was pressed into service, weeks ahead of schedule, in response to the V'Ger crisis, once again under Kirk's command.

In addtion, I seem to remember reading from somewhere that Scotty wanted to add newly developed warp engines to the Enterprise, and upon adding the engines, he found that the pylons needed to be upgraded to hold the engines. So what began as a simple upgrade slowly morphed into a large redesign. I also belive that when this refit was done, that the Enterprise was reclassedifed from a Constitution class to what is now known as the Enterprise class.

Yes, this is the same Enterprise, NCC-1701 though, since the refit was so extensive, there is an argument to be made that this is a case of a broom being given both a new handle and a new brush!

In release order, the first totally new Enterprise we see is the NCC-1701-A, the renamed Yorktown, at the end of Star Trek IV.

I was fortunate to serve aboard the USS Aspro (SSN648) in the 1980's. She was in docks for a refit at Mare Island, and was completely gutted to be given upgrades for nearly every system including her propulsion, but her shape remained intact. This however isn't how surface ships are affected by refits.

The Aspro's shape was critical to her sub-surface capabilities whereas surface ships aren't beholden to their previous shape. Especially in their super structures (i.e. Guns to missile batteries can alter a silhouette to some degree, but deck rearrangement like the Intrepid obviously have larger impact to its silhouette). Ultimately, both vessel types are still considered refits.

In any case, my point is that it really depends on the ship, mission capabilities and even the politics of governing bodies, so I don't think the future would be much different.

All with a grain of salt of course :)

You'd think that in a refit the major structural features would be largely unaltered e.g. the main pressure hulls. To make any significant changes there would amount to a rebuild rather than a refit. But, they called it a refit in dialogue, so it's a refit in canon.

Yes, you can see changes in pretty much all the ship's lines, including what would be pressure hulls. Since the original series shooting model remains unaltered to this day (now a museum artifact), the movie used its own models, so the new model wasn't just a re-dress of the old. According to Wikipedia, production staff wanted to change the look of the ship and got enough of their way that the result looks like far more than a refit. Same goes for all the interiors - all the interior spaces represented on-screen whether by actual physical sets (mostly) or matte (I think there's one or two).

Since TMP Enterprise bears the same NCC-1701 number as TOS Enterprise, it must logically be the same ship in spite of all the changes, else it would have been assigned a new number (e.g. NCC-1701-A, which was given to the ship the crew flew in a few of the later movies e.g. ST:The Final Frontier). So, it's definitely not a "new" build, and as I said above, it's referred to in on-screen dialog as a refit, so that's what it must be.