Iron Maiden's Number Of The Beast gets my vote. Or Sgt Pepper's Lonely Hearts Club band with Paul's replacement.
Well, yes, Discipline of course...not that I would describe it as progressive rock but we'll let that pass. But this one. This had four new members in Wetton, Bruford, Muir and Cross.
Heaven and Hell is probably my favorite. It's easily one of the best Black Sabbath albums and one of the better metal albums of the early 80's. King Crimson has had a few of these over the years, but I agree with earlier posters that Larks Tongues in Aspic and Discipline are the best and most striking. Though Rush still had a long way up from their "second debut" Fly By Night is still a solid album and one of the most necessary transformations. I also really love Nursery Cryme, The Yes Album, Days of Future Passed, and a few others that I don't consider "radical" enough to qualify.
One I like that hasn't been mentioned yet is Gazeuse! by Gong. Their previous album, Shamal, already featured them trying to move past the departure of Daevid Allen, but it was the further lineup changes on Gazeuse! that transformed them into a whole different band. The addition of Allen Holdsworth and Pierre Moerlen's fleet of co-percussionists purged all lingering psychedelia and fully embraced the jazz-rock gamelan sound that was only hinted at on the band's most famous releases.
Captain Beefheart And His Magic Band-keep John "Drumbo" French, Jeff "Antennae Jimmy Semens" Cotton, and the Captain himself, then switch Alex St. Clair and Jerry Handley out for Bill "Zoot Horn Rollo" Harkleroad and Mark "Rockette Morton" Boston, and we end up getting Trout Mask Replica
One that I've just remembered as I've been playing Futurist Manifesto is Be Bop Deluxe. Futurama is Bill Nelson plus two new members having ditched the musicians who were on Axe Victim. Never did understand how "Maid In Heaven" wasn't a hit...
A lot of the obvious ones have been mentioned...so,sorta a contradiction in your premise,but I think Rush should qualify...there is certainly a difference between that first record and when Neil Peart joined on their sophomore release
Anthrax, Spreading The Disease: Singer Neil Turbin and bassist/cofounder Dan Lilker are switched out for Joey Belladonna and Frankie Bello, and the classic lineup of Anthrax.
I wrinkled an eyebrow at this at first but actually, you're right. As much as it was circumstantial, a trained multiskilled musician was an ideal choice to fill the space in the group's sound created by going down to one drummer and, of course, he wrote Spoilt Victorian Child and most of Paintwork.
