I thought TNG was converted yonks ago, considering TOS was done way back.
Would be interesting to see how better it looks.
They can't as the TNG masters are ALL on Digital Betacam tape (with about the same resolution as a 480P reolution standard definition DVD)
TOS was easy as it was all filmed on actual 16 mm film, and they just needed to rescan the prints with a HighDef digital scanner; and digitally redo the visual effects (and TOS effects shots were sparse and short anyway due to 1960ies visual effect production costs, so it wasn't that hard to redo those shots.
The only way they could use the existing video master fpr TNG would be to try and upconvert - but they did a test on a few episodes and the results were not good. To get TNG remastered in HD they need to find the original film stock - scan that with a HighDef scanner - recreate a Highdef master (using the esiting video masters as a guide for live action - and also digitally redo all the effects (the video effects were done and created on video directly (IE shot using the digital betacam cameras), and finalized and transfered directly to the video masters.
So, there's a lot more work involved in creating true HD masters of TNG episodes in addition to there being 90 or so more episodes to do than TOS had.