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?
Replace or repair machinery, equipment, and fittings in (a ship, building, etc.).
Raise (something) to a higher standard, in particular improve (equipment or machinery) by adding or replacing components.