Well, I wouldn't have a big problem if they ever decided to remaster some of their classic episodes. I am putting emphasis on "some", because as [MENTION=86919]Trab Pu Kcip[/MENTION] stated, not every non-HD era episode can work reanimated with HD animation. I believe that maybe seasons that they didn't really bother with the animation, like seasons 5-6 do need remastering and even episodes from the Scully and early Jean era need that too. However, like I said, not every episode can feel the same after remastering the animation. It can take away almost everything that made it a good episode: The humour, the aesthetic, the vibe, the general feeling etc.
But I believe that this is a little pointless to discuss since...well...the show is over 30 years old and in those 30+ years, fans of the show have watched a lot of the classic era episodes (The hardcore ones all of them, heh) at least 2 or 3 times due to reruns. It would be weird if say, someone decides to watch "Lisa's Substitute" one day and they suddenly bump into the revamped version of the episode. Actually, maybe this isn't the best example I could give...but you get my point! Not everyone reacts in the same way to sudden changes. Not everyone is a hardcore fan of the show! Not everyone does regular research on possible news and changes to the show and not everyone keeps up with it. Some fans might like those changes, others not.
Some things are better to be left alone and most of the classic episodes of the show is one of them!
As for re-recording the dialog or recasting...I believe that would be a really bad idea, especially now! There have been countless discussions on this board about the voice actors and actresses having aged and the characters not sounding like themselves anymore. Do you remember how everyone freaked out about white actors no longer being allowed to voice characters from different races, even they have been voicing those characters for 10, 20 or 30 years? I personally wouldn't like abrupt changes without a reason (At least in this recent controversy and the Apu one, there is a law preventing them, but not here) and neither would a lot of fans for the same reason.
Honestly, would you feel like you are watching a classic episode if you heard Marge or Bart's lines re-recorded with the current state of their voice actresses' voices? Or if Alex Borstein did the re-recording of the lines for Lisa? Or if Seth MacFarlane did most of the male voices on the show, instead of Dan Castellaneta and Hank Azaria?
I know I wouldn't!