For some reason Linda's singing sounds different to me each time I hear the theme song. I've noticed subtle changes in her vocal delivery. I'm sure the theme song changed each season, but sometimes it sounds like a different arrangement during the SAME season.

The theme song arrangement and vocal arrangement changed each time during the first six seasons. For seasons seven through nine, the theme song stayed exactly the same as the season six theme.

Furthermore, in season two there were two different similar arrangements broadcast. In season three and going forward for the rest of the series run, the lyrics changed quite a bit.

The ending theme stayed the same for seasons one and two, but changed each season during seasons three through six. During the third season a different ending theme was boradcast only once, which included vocals by Linda Lavin. For seasons six through nine, the ending theme was the same as season six.

There may have been some details that I missed, but this is what I can think of off the top of my head.

You have a terrific memory! I'm currently watching season 2, and I've noticed subtle changes in her vocal arrangement but I've been too lazy to take detailed notes.

I notice the song gets HAPPPIER as the series goes on.

I thought in the first season the song had a very sad meloncholy sound to it, then in the later seasons it's very happy and peppy.

I noticed another different musical arrangement during the closing credits in the Season 4 episode, "Vera Robs the Cradle" : It was different than the theme song, but similar in a way. I liked it. It was jazzy and sounded like a trumpet or horn.