. . .The main reason why TOS was capable of being bumped up to HD is that fact that it was shot on 35mm film (i.e., the source was already HD). The blu ray versions of the show will let you watch the original effects shots.
Until the advent of hi-def video,
all TV shows that weren't live or videotaped were shot on 35mm film. 16mm telecine prints were made for broadcasting. The
Enterprise flyby shots in Trek TOS became grainy and color-distorted through multiple generations of printing, as the same stock shots were used over and over and recomposited with other elements.