20,000 Leagues is being shown on DirectTVs DHNet hidef channel this month and I happen to catch a viewing this weekend. I can't say what they're showing is the same source that will make it to Blu-ray but it *is* definitely a new HD master and there are significant color timing changes and image/frame composition changes as well. There also appears to be a (un)healthy amount of DNR and minor edge enhancement going on as well.
For starters, the opening credits sequence, those background curtains which originally had a rich reddish orange color (which was significantly toned down for the dvd release) are now almost colorless until the blue water reflection is projected on them as the credits roll (which are now in crystal clear light blue). Colors overall feature a significantly desaturated look throughout the movie. Underwater scenes have a much more flat, "blue" look to them. There's numerous darker scenes which have black crush.
Real issues become apparent during the attack and sinking sequence of the "nitrate" slave ship at about the 1 hour mark. In the original movie and all previous home video presentations, as the Nautilus waits in the lagoon for the ship to leave with the tide, the sun sets over a warm, orange cloudy sky - however in the new HD print, the sky and clouds are now tinted blue (although the sun is still warm yellow/orange) and the lagoon waters reflect a darker bluish hue. When the Nautilus is about to ram the ship and there's several fast cuts between Nemo's face and the ship from his POV, in the original film the ship was "stretched out" in the frame *on purpose* to give it more of an imposing size on screen even though it does make the ship look out of proportion - in this version, the brief shots of the ship have been zoomed in and the proportion corrected. It *may* be a better choice compositionally but it's not what the original scene look(ed) like.
There's several other notable issues like the above throughout the movie where apparently the restoration team either didn't understand the director's intent and thought they were "fixing" things that weren't actually issues or the decision was simply made to correct what they perceived as problems in individual scenes for "today's audiences."
The audio is problematic as well. Much of the movie, the soundtrack is center speaker-specific but at really odd times, the soundtrack will fill the L/R and rear surrounds.
Obviously, a HD broadcast often isn't a really good barometer of what an impending Blu-ray release may look like, but in this case regarding one of my favorite films, it may be a troubling indicator.