Carlos Santana. His guitar playing remained exemplary, but, from the Supernatural album onwards, in the words of CR magazine, he became a guest musician on his own albums.
Budgie tried a West Coast sound in an attempt to break into the American market.
Judus Priest, once they had exhausted the Atkins material, became a different band.
Rush moved away from their roots as Led Zep soundalikes and, again, from their best style (2112, Kings, Hemispheres).
Hawkwind moved away from their roots as a heavy space rock band, after Lemmy's departure. They returned periodically.
Black Sabbath had me worried when they began using overdubs on the guitar, then Ronnie James Dio joined.
Jethro Tull moved away from the folk inspired heavy rock on JTull.com, having already moved away from the earlier blues rock. I read a review where the writer described them as being like his dad dancing at a wedding.
Groundhogs tried two guitars for a while, which seemed odd because Tony McPhee was like two players rolled into one anyway, but it was good.
Deep Purple became heavy funk for a while on Come Taste the Band, and it was pretty good. Ian Gillan tried jazz rock on Clear Air Turbulence, but never returned to that style to my knowledge.