ven the purists do have to admit that the Enterprise-D by Gabriel Köerner as it appears in Star Trek Enterprise's finale looks marvelous in comparison to the studio model, just because of the way the lights and shadows hit it, and the camera angles you can easily achieve with CG.
Yeah, just make sure the saucer's impusle engines aren't on. Oh, and you don't try doing a fancy camera move beyond the furtherst point in the model, making it appear the ship is flying in a snow storm.
I think an upconversion of TNGs effects could work, they're certainly working over on TOS, and since the original effects are, indeed, on videotape (I actually think they were filmed but then finished on tape and that what was used for the final cut) it'll certainly need the upconversion to survive in an HD world.
But the question you have to ask is: Is it worth it to the studio?
TOS is a classic in its own right which is why it got the upconversion people still watch it since it is a classic. If the same can be said for TNG then it can get the upconversion. But you have to make it worth it to the stations to buy it and air it for it to be worth it to the studio to do it. Which means, since TNG remains the second most popular series, it would be unlikely DS9 and Voager would ever get an upconversion becuase their popularity with any networks isn't as great as TOS' and TNG's.