It doesn’t have to be sudden; it could be over time, and it doesn’t necessarily have to be a negative change.
Syd Barrett is the first one that comes to mind. Jim Morrison seemed to change a bit, maybe doing things he didn’t want to do, but having external pressures, as well as his heavy drinking which I read was because he couldn’t sleep, and wanted to shut up the demons in his head, but I don’t think it was as drastic as Syd’s.
I believe Roger Daltrey was originally a violent bully who ran the Who (High Numbers at the time) by beating up the others. Townsend and Entwistle finally laid down the law: Pete would be the leader and if Daltrey didn’t accept it, he was out.
Daltrey agreed to curb his temper. Turned out to be a smart move.
What I’ve read is that Lennon had a mean streak in the beginning, but any blowups were over with quickly. Harder drugs and alcohol (and Yoko) turned him into a dick.
Michael Jackson. In addition to who knows what was going on in his mind, he apparently got hooked on pain pills after the infamous Pepsi commercial fire incident.
Nikki Sixx maybe. He used to be a very unstable asshole but as he got older and dealt with his issues he seems to have gotten more low key. I believe Vince Neil said he could work as a therapist now. Considering how fucked up he was in the 80s, thats a pretty dramatic change.
Mike Oldfield went from an introvert to an extrovert after 1978 when he joined the Exegesis group to try an correct a “bad birth”
Michelle Shocked, Cat Stevens and Annabella Lwin all had fairly extreme religious reawakenings.
Yep, this was mine too.
j
Purportedly arch-asshole Johnny Ramone( Cummings )mellowed out considerably once the band dissolved for good and he no longer felt he had to compete with people. Those who knew him well said he was a bit more of a pussycat towards the end of his life.
However apparently there was always a tiny element of theater to his irascibility anyway. He rather enjoyed the image of being a hardass tyrant. Chris Stein of Blondie for one has said he always thought he could be a nice enough guy. Just not so much with his band-mates :).
Did Cat Stevens change much? He always seemed like a nice, relaxed guy.
Billy Joel was a very nice, mellow guy when I met him when he was just starting out. After his BIL screwed him out of his money and Billy got famous, he got nastier.
And screwed the rhythm section, and then fired them. Doug killed himself soon, too.
Roger Waters is apparently a much nicer person to deal with now than he was in the old days.
Eric Clapton has been trying to decades to become a black (am I allowed to say that?) bluesman.
So true! I saw him in concert, and he mostly did covers of old blues standards. The best song he did was a cover by Bob Marley 
He’s still playing the same scales over and over and over.
Bob Dylan I think?
I haven’t seen him since his “conversion,” but he changed his religion to the Muslim faith and his name to Yusuf Islam, and mostly dropped out of the music scene. I’d say that was a significant change.
Lou Reed “transformed” himself a number of times.
In the 60s he had the reputation of being a Class-A jerk, and by the 70s his jerkness and arrogance reached the point where he didn’t even want to talk to people. He created an image that he was seedy, drug-addicted, and bisexual.
Then 1980 came, and Reed did a complete makeover. He cleaned himself up, ditched the homosexuality & drugs references, got married, and declared he was a regular ol’, hetero guy. He became more friendly, more mainstream, and even did some commercials for Honda motorcycles.
He is back doing music now .