This is a really interesting topic

Aside from what has been mentioned already - namely The Eagles and The Beatles - a couple others come to mind for me.
When Brian Johnson took over as AC/DC's singer is really when they started releasing the music I came to know and love. I've never been super into the band, but all the music I'm most familiar with from them is in the Brian Johnson era.
A lesser known one would be Flaming Lips. A lot of people who listen to them don't even really know they used to have a different lead singer. Wayne Coyne took over as the lead singer in 1985 after the original singer left the band to pursue other things.