I have started to watch the third season on DVD. I am wondering if the show had been renewed before the end of filming of the second season, because the last four or so episodes seem updated compared to the rest of third season. Judy started wearing a hair band. In "The Phantom Family," you see the first use of what I guess could be called green negative film, as you see the characters get zapped and the whole scene changes color. Also, the equipment in the lower level living quarters is more vibrant and elaborate and looks new.
At the start of the third season, the characters wear their silver space suits and Penny wears her hair long, but minus bangs. Although there's never a mention of it, Don and John must have gotten enough nutronium to allow a sudden liftoff. Then they go to the space prison, which has a bright blue elaborate complete setting--the black backgrounds of the interiors of previous alien ships is gone. There is also, of course, the new opening credits, the one everyone likes better because it is more exciting.
The second episode is "Visit to a Hostile Planet," when the crew goes back in time to 1947 Earth. Much of the episode is filmed outdoors, rather than the alien planet soundstage scenes. The crew still wears their silver suits, except for a brief scene when you see the new outfits and Penny's hair is pulled into a bun. This episode "feels" like it should have been the season opener.
I am wondering if the episodes were switched. In the opening of "Space Prison" the Jupiter 2 lifts off but then is thrown into an erratic course because of a comet and a supernova. In the opening of "Visit" the Jupiter is traveling at the speed of light but it is a chaotic ride. Perhaps the original scripts had the comet and supernova creating the erratic course?