One final time: (We wish)
First of all "remastering" means going back to the original filmstock, cleaning it up and then putting out there.
Secondly: TNG's special effects were done on videotape and furthermore the series was taken from film and transfered to videotape for editing. (Granted, high quality videotape.) So this means they filmed everything on film, sent it to the editor, who picked the good takes/shots, etc. and pieced the episode together onto videotape.
This means that to "remaster" TNG someone has to go back to the film and completely put the episodes back together again!
This means finding all of the shots and takes and whatnot that were used and pretty much make the episode over-again from scratch. It's not impossible but it'd be a pretty huge undertaking. There's only so much resolution that you can get out of videotape and HD quality isn't in that particular deck.
"Upscaling" is over emphasized when it comes to TV and DVD/BD players as it sort of makes the presumption that the player is taking the material and "somehow" knows what's going on between two pixels and give you that added detail. It can't.
It can only make a "best guess" and make the picture fill the screen rather than being in a box. It cannot add detail and resolution that isn't there which is what HD is all about.
TNG's been rumored to get a "remastering" for sometime now but it'd be a huge, expensive, project for Paramount to take and it'd just be a question if it is worth it in the end. I'll admit I'm not sure if the effects were done on tape, but I do know it was edited on tape which really limits things.
It can be done but Paramount has to be willing to invest quite a bit into it and fans will have to be willing to pony up for another set of over-priced BD sets. (There were enough complaints about the price of the DVD sets.)
TOS it was easier to "remaster" since TOS was recorded and edited to film so there was a lot more there to work with.