Hi,
The American TV series "Monk" had a wonderful instrumental theme for Seasons 1 (and possibly 2), by Jeff Beale, but then, later seasons went and used a crappy "song" by Disney/Pixar supremo Randy Newman, entitled "It's A Jungle Out There". I loved the original, as it fitted the show, and the characters better. The "song" they use now, is horrible, and doesn't gel with the title credits at all.
"Happy Days" originally had the song "Rock Around The Clock" by Bill Hailey (And The Comets), for the first season or two, and then an instrumental theme briefly, (if my memory serves), before the famous "Happy Days" theme we all knew and loved! The titles also changed with the "Happy Days" song, from shots of the show, to shots of character Potsie singing the song itself.
"Airwolf" had several variations of the classic music by Sylvester Levay. They were almost the same as one another, except for their duration, and sequencing of certain sections of the music. The theme was sometimes about 2 minutes in length, to as short as 50 seconds, depending on the episode and the season in question. The same issue also occured for the end-theme credits - sometimes it would be the full-length theme, whilst at others it would be severely cut down.
"Quantum Leap's" theme music was always good, except there was one occasion when they did use a really terrible piece of instrumental stuff for a few episodes, around Seasons 3 and/or 4. I can't recall how it went, or why it was changed, but I do remember it being bloody awful! I'm pretty certain however, that US fans complained in their droves, and Donald Bellisario replaced it with the classic theme that the fans preferred.
"The Bill" has had various theme tune changes in recent years, but none have been as good as the classic mid-1980's version, that started off with the sound of the police siren, followed by squealing car brakes, and then the instrumental theme. All the other versions have been awful, especially the newest ones.
"Desperate Housewives" has had its main theme changed, for Season 2, which is a shame, as I loved the Season 1 credits scene, and the homage to Andy Warhol pop-art! It's always dissappointing to have a great piece of music changed.
I can't think of any others that have changed, although I'm sure there are plenty.
Pooch