Voyager as far as I know doesn't need redone FX shots (at least not for about 90% of them).
Merely cleaned up/sharper image to eliminate the blurriness and raise it's quality.
They could make some minor animations to eliminate the 'stock footage' that was used on a frequent basis... but even that would entail minimal effort.
TNG would require more work since it contains planetary bodies that weren't really that great looking and as such would have a bit more work cut out for the FX department (but nothing that cannot be handled in a relatively short period of time).
DS9 is quite similar to Voyager in a sense that it requires little to no new FX but could use a retouch in higher quality and sharper image and maybe eliminate some stock footage that was used too much.
Voyager and DS9 would probably require the least amount of work compared to TNG.