Before anyone says my examples are not "hard rock", the article doesn't mention that distinction in the title so ...How could you leave out the "Doobie Brothers"? Their sound was completely different before Michael McDonald, then during Michael McDonald, then after Michael McDonald. And, what about Chicago? They changed after Peter Cetera, though I don't know if that was more a reflection of the evolution of music at the time but they got super ballad-driven in the late 80's.