Fleetwood Mac.
Genesis.
Eric Clapton. Listening to his 80s albums you'd be puzzled as to why this guy is revered as some blues guitar genius.
Rush. I actually like latter era Rush, but it's a far cry from the hard prog rock of the 70s.
Bee Gees. Being only familiar with their disco music for many years I was amazed that they were actually a great vocal harmony pop group in the 60s and early 70s.
Queen to a degree in the 80s. I bet "fans" who know them by I Want To Break Free, Radio Ga Ga and A Kind Of Magic would be surprised to hear their early prog metal albums.
I personally don't think Aerosmith have changed as much as some seem to think. Or maybe those people have only heard their later singles. They were certainly grittier in the 70s, but I never thought that their post '87 records were *that* different.