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.
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.
Not the case, sadly. Voyager and the later seasons of DS9 used extensive CGI which won't hold up in high definition. DS9 used mostly motion-control until the sixth season, so that can be remastered similarly to TNG. Later DS9 and all but the first couple of seasons of Voyager would need completely remade space sequences, unless they just upres it, which would look pretty crap.
With Voyager, the stock shots and most of the model work used in the first couple of seasons would look fine, but all those iconic scenes in Scorpion, Timeless, Equinox and Endgame would suffer unless remade.
I think they'll get around to it eventually, as it makes sense for CBS to have a full Star Trek HD catalogue to offer for syndication. But it's a bigger job than TNG-R.