TOS was completely done on 35mm film - editing was done by cutting and splicing film, physically; and phaser shots were done by painting on the film, physically.
The space FX were redone for TOS-R because the composited model film was in a horrible state - in addition to the multi-generation compositing which shows up in SD, and would have stood out horrendously in HD
TNG was wholly shot
on 35mm film, including the motion-control models which made up most of the space FX, but all editing was done on 480i NTSC video. All phaser shots, replicator effects, LCARS shots, any glowy stuff, etc - even that net from Encounter at Farpoint - only exist as 480i video
. In addition, the episodes weren't even preliminarily cut on 35mm film.
To do a HD version of TNG (or "TNG-R") would be possible, but the episodes would need to be completely reassembled from the 35mm camera negatives, and every effect other than the motion-control models themselves would need to be CGIed from scratch (which includes the compositing
of the models into one image). A glimpse of how this would go can be seen in the footage reused for ENT's finale, where a replicator shot had to be remade.
To do a HD version of DS9/DS9-R would be even trickier, since all the TNG stuff above applies (plus add in the changeling shapeshifting as an additional source of Stuff To Do), but as the series went on they started making more and more use of CGI for the ship models - often composited with models in one shot when you see a fleet - and all
of that would need redone [and VOY heads even further down the CGI route].