I have a Lost in Space
CD that includes third-season re-recordings of first-season music which is available on other CDs. They're basically just different performances of the same pieces, with the only differences being slight variations in how they were conducted/performed/mixed, the sort of thing you have to be pretty familiar with the cues to pick up on. Things like slight differences in timing, or different relative loudness of two instruments playing at the same time, or the lack/addition of some slight imperfection or flourish in a musician's performance. I'd imagine the TOS re-recordings are much the same, since I know that show's music better than any other music I've ever heard, and I've never picked up on any blatant differences.