This has been discussed before, and I believe the official word is that releasing TNG in high def would actually be a much larger undertaking than TOS. The reason is that the special effects for TNG were shot on video, not film. And even though the live action was originally shot on film, and therefore is high def quality, it was all immediately transferred to video for editing, compositing with special effects, etc. So the completed shots only exist on video.
Therefore, nothing from TNG currently exists in a high def format. You'd not only have to re-do every single special effects shot, but you'd have to go back to the original negatives of all the live action footage and start from scratch.
Is it possible that this could be done? Sure, but it seems unlikely given the cost involved.