Bands that took a major change in direction

[Stickleback]

The Gathering

[emperorken]

Pallas was one of the best neo-prog bands around. For their last album in 2011, "XXV", they not only dispensed with their great singer Alan Reed, but they completely changed their sound to one of mostly hard rock. Quite a disappointment. If I did not know it was Pallas, the album would be unrecognizable as them.

[Big Ears]

The Cult, from goths to hard rock

Queen, from hard rock to gay disco

[Yodelgoat]

Hobbit - First album, Prog and Middle Earth, second album - Butt-Rock, no middle earth - they even start the 2nd album with a Middle earth intro for about 20 seconds, then they say: "Frack this!!...Lets ROCK!" and proceed to bleed mediocrity. I think the label won, but then everyone lost. Sad story, like Golum's.

[JIF]

The Cult, from goths to hard rock

Queen, from hard rock to gay disco

Was the gay part really called for? Their singer actually was gay. That's like calling Judas Priest gay metal.

[Big Ears]

Queen changed from the hard rock of May and Taylor to a style of music played in gay nightclubs in Germany, frequented by Freddie Mercury. Deacon played a large part, with the music being bass-dominated, as he and Mercury tended to support each other. May and Taylor have admitted they went along with the style, despite it going against their better judgement. The OP said 'major change', so this would qualify as it occurred in the space of one album, Hot Space (although The Game had a pop sound with synthesizer).

With regard to Judus Priest, I don't care for the term 'metal' in any case. Certainly, in a couple of albums, they went from being a great heavy rock band to a pop group.

Mercury and Halford deserve credit for being open about their sexuality, in a predominantly macho-genre, at a time when it could effect careers. It's as brave as Graham Bonnet and Bruce Dickinson having short hair.

[Scrotum Scissor]

Italian fusion band Dedalus-compare their debut with the second album-two different things.

Good call. And the same goes for early vs. mid-period to later Franco Battiato - practically different approaches to music altogether.