Depends what resolution the visual effects were originally rendered at.
I know the first three seasons were shot on 35mm in a 1.33:1 aspect ratio (full screen). The final season was shot in a 1.78:1 aspect ratio (widescreen). I believe it was still filmed in 35mm, not on High Definition Video, but I could be wrong. This doesn't really matter--35mm has many times the resolution of HD.
It's possible the first three seasons were protected for a widescreen frame, but I'm not sure if they were (and the visual effects probably weren't prepared for such a frame, and would have to either be cropped or redone if these seasons were made available in widescreen).
Ultimately, it is the visual effects that will mean if Farscape
is ready for an easy transfer to HD or not.