I guess you don't want any Trek to be suitable for rebroadcast in the future, then.
There must be other ways to accomplish that. Messing up TNG like they did TOS is neither acceptable nor necessary. I'm sure they could do a decent upscaling job that would satisfy the viewers.
And even they are not HD, then they are not ... where is the problem? People are still watching old black and white movies, so why shouldn't they watch Trek in SD? Personally I'd rather watch it super low resolution than ruined with new fx.
Sacrificing the original TNG (or TOS or whatever) for HD is like trading pest for cholera, so they might as well stay with the original.