The problem is, DVDs have got 'artifacts' due to their compression and the MPEG routines used to put them onto the DVDs, which even an upscaler won't get rid of. In this regard, even VHS has got a certain clarity that DVD doesn't...
In terms of these compression issues, that's one area where even taking the existing "master copies" and simply transfering them to Blu Ray without any further remastering would
still result in a better presentation than what's on the DVD, because they'd be lossless transfers of the broadcast copies. Blu Ray doesn't suffer from compression artifacts because the actual capacity of the discs is much greater than a DVD, so they don't need to lessen the quality of the picture and sound in order to cram them onto each disc. So long as these theoretical Blu ray releases used the broadcast master tapes as a base rather than the existing DVD files, then logically they'd already be a whole generation better quality than the DVDs are. Sure, they wouldn't look as good as TOS-R or TNG-R do. But they'd still be an improvement on the DVDs.
The best upscale results are gained from the combination of a good Blu Ray player, a good HD television, the correct settings, and the use of HDMI cables (rather than the red-yellow-white 'composite' cables that DVD players typically use). All of these things can lead to DVDs being upscaled satisfactorily.
In any case, upscaling a DVD won't get rid of the DVD compression artifacts. In a best case scenario it'll compensate for it, but the results will vary depending on the quality of the DVD to begin with. It's a matter of pure mathematics as well: you simply can't take a lower resolution picture and magically turn it into a higher resolution than it already was. The picture data just isn't there.
But like I say, a good setup will help a multitude of sins.
