shot many of its visual effects on 35mm film. These shots would simply have to be rescanned and recomposited
If Star Trek: The Next Generation is ever brought to HD, Paramount may elect to recreate all of the visual effects using modern CGI techniques. But they wouldn't have to.
is correct in that it could be done if cost did not matter but it does matter.
It would not be cost effective to telecine all the Enterprise-D exterior model shots for compositing.
Any live-action ship interiors they would do the visual effects over the filmed footage.