They aren't changing the aspect ratio, but I don't see why it should matter even if they did. This isn't a STAR WARS situation where people can't get the original versions the way they first viewed them. ST:TNG was originally broadcast NTSC standard-def video - if you want that experience you already have it on DVD.
Taking ST:TNG to HD is, by itself, a significant departure from its original presentation. Even if they did change the aspect ratio (which they aren't) so what? The moment you say, some changes are okay, but others aren't you've departed for Lucasville.
I don't expect the HD version of this series to be exactly the same as originally broadcast, I fully expect there will be many changes - some subtle, some obvious.
Personally, I'm all for it.