Quality of restoration of older TV shows and movies varies from studio to studio, Warner Bros for example has done restorations of both Gone with the Wind and The Wizard of Oz and the results are stunning especially when you see it on TV as they provide the newer copies of those films. In regards to TV, Friends is one that comes to mind where the newer copies look better than they did originally.
Yep - it all depends on whether you have decent source material, and the budget to recreate EDLs if they no longer exist and the show was originally shot on film and cut on tape.
ISTR that Star Trek : TNG used some image recognition stuff to match re-telecined HD rushes to the original SD edit masters as the EDLs didn't survive.
If you don't have the film elements and only have SD VT
copies, then up conversion is the only route.