I'd love to see this stuff in HD. As already pointed out there are FX problems. It may not be all that bad b/c the models were filmed on film (that sounds redundant). It still seems like it might be a little more expensive than I think Paramount/ CBS would be willing to spend.
I'd really love to see all Trek upgraded for HD. I don't know if it will happen though.
Well, basically what it would need is for all the post-production to be done over (the individual model elements exist on film, but "kitbashed"/background-only models probably wouldn't stand up to the increased scrutiny, and all the CGI work would need redone). You're right in that that makes it a matter of cost. Who knows if they'll ever think it worth it? (probably not unless they start to struggle to sell the shows in SD at all