Actually a lot of older movies are being put on to DVD that were not shot in HD. If you do your homework you can find movies on Blu-Ray that go back all the way to the 80s, which were most certainly not shot in HD.
Motion pictures are typically shot on film, which has a higher
resolution than HDTV. Films from as far back as the beginning of motion picture history, if they've been preserved or restored to their original condition, will continue to look absolutely gorgeous
in HD release on Blu-ray.
The issue is that, starting in the 1980s, video proved to be a much cheaper, more efficient method of storing and editing motion pictures. Video, however, has a drastically reduced level of resolution when compared to film or
HD. In the world of television series, though, this didn't matter as much because television resolution has always been fairly low. This is why Star Trek
(which was produced on film in the 1960s) is seeing a Blu-ray release, whereas The Next Generation
(produced on video in the 80s and 90s) will require a complete re-editing from the film source in order to achieve HD resolution.
The question being asked in this thread is whether or not Farscape
was shot or edited on film or video. If it was shot on film but edited in video, there's a chance it could be re-edited in HD but it would likely require a complete revamp of the visual effects (and the studio might find this cost prohibitive for such a cult series). If it was edited in a medium that was a high enough resolution, though, such concerns would be moot, and a Blu-ray release would be as simple as striking a new transfer.