The VFX elements for TNG were mostly kept, so it was mostly new post-production - it was DS9 that got bit hard, since there was much more CGI in the later seasons making an HD remaster too expensive.
They may have gone back to them for the Blu-ray release of ST:TNG. If you haven't seen them, the picture quality is amazing - far better than the DVD releases. Word is that CBS/Paramount said the cost was such that they'll never do it for the other Star Trek TV properties.
