The plot for this one, featuring one of Star Trek's most unique guest stars, would infamously be recycled for the first Star Trek movie. (The film arguably does it better, but the episode gets quite a bit of mileage out of the premise, leaving little on the table for the film to elaborate on). In contradistinction to Star Trek: The Motion Picture, however, this episode is a bottle show.
The guest star of honor is Nomad the robot, played by a prop (three, actually) and voiced by Vic Perrin, the latter doing such a fine job that his work in the episode is probably more memorable than his appearance in the next episode, "Mirror, Mirror" (as the leader of the Halkan council). But "The Changeling" is a Kirk episode all the way, and William Shatner works hard to prevent the bucket of nuts and bolts from upstaging him, though he gets help from a tight script with a witty ending. (Uhura also gets some nice moments, singing a song and establishing her native language; it's just too bad the episode doesn't feature a new musical piece, instead, lifting one from "Conscience of the King").
There is an undeniable appeal to the idea of a machine gaining consciousness and seeking its creator, which is probably why the plot device is used again; but perhaps the highlight of the episode is seeing director Marc Daniels in a photo playing the distinguished looking Jackson Roykirk!
Remastered: With no planets and few visuals needing touchups, there's little for CBS Digital to do here, though they upgrade the visuals of Nomad's attacks from space and the explosion at the end. (Personally, I prefer the original explosion, a practical effect that looks more realistic than its digital replacement). The new CGI beauty passes of the Enterprise are pretty much copies of the original footage.