Opening credits for a lot of shows change from season to season. One example is “The Unit”, which replaced its entire theme song. The M*A*S*H folks may have re-recorded the theme song for various reasons; too long, too short, bad fidelity of the original recording or whatever. The composer or arranger may have decided to take the re-recording opportunity to make a slight adjustment to the music, perhaps to “fix” something they didn’t like.