There's so many. Sometimes a change of direction is'nt a bad thing , but , other times , it's fatal. Some of the bands I could recall probably are'nt even well known enough to warrant a mention , HERE.
The Saints - In the Late 80's , I passed on seeing them , as they were starting to sound like REM (As was nearly everyone.).
They recovered , many years later , with two boss Punk - oriented albums , and , I'm told , some great shows.
Plan 9 - Eric Stumpo stepping down as lead vocalist (When he has such a great voice.) , and bringing in a younger guy with that melancoly Echo and The Boringmen kinda sound , did'nt seem to musically make sense , nor make them any more popular.
Pandoras - God Rest Paula's Soul...... I admired her for her 'tood as much as her music. Their first two albums , with two different lineups , are still great, but , when they got into that "Metal" thing with "Rock Hard" , they lost me. Some fans might have dug it , anyway , but , I thought they could have done much better.
Pink Floyd - Syd leaves. A year or two of good material spread out over parts of their next few albums. I'm being kind.
Fleetwood Mac - Peter Green leaves. The band puts out at least one very good album afterward , but , it's the beginning of the end. We all know , in 1975 , they jumped a shark that would have devoured The Fonz.
Sex Pistols - Glen Matlock leaves. The sensationalism is just beginning with Sid Vicious in tow , but , the music takes a dive. It was'nt entirely their fault , when they were'nt allowed to gig in The UK , and a tour of The American South and San Francisco almost did'nt happen , and meant their undoing , anyway. One great album , with an uncredited Matlock on board as a "Studio Bassist".
The Beatles - Stu Sutcliffe dies suddenly. Pete Best is fired. Now , NO ONE in the group can grow a proper quiff.