(Bands/Artists in alphabetical order)
1) Alice In Chains
I was shocked when I found out Alice In Chains started as a Glam Metal band, with the name Alice N’ Chains then. But unfortunately they chose to became the Grunge clowns they are known as. Listen to their demos as Alice N' Chains, and throw the Alice In Chains crap away.
2) Michael Bolton
I believe Michael Bolton is the most tremendous example among the list, and also the saddest one. In the 70s he released two fairly good Rock albums, Michael Bolotin and Everyday Of My Life. In the mid 80s he made a change to his sound, and released two fantastic AOR albums, his self-titled album in 1983 and Everybody’s Crazy which are definitely the best of his career. But since then, he changed even more to the worse. He chose to become a Pop crooner in the late 80s. Although his commercial success was bigger, his quality in music was decreased dramatically.
3) David Bowie
David Bowie was known for changing both his appearance and his musical style very frequently. Yet, no matter in which genres he has been involved with, truth is he was always second best than his rivals. I guess he should’ve tried to master one genre instead of changing musical direction as frequently as he used to. But anyway, I believe he would remain in history mostly as a Glam Rocker since that was his most successful period (first half of the 70s), with his New Wave era (second half of the 70s) coming second.
They were one of the first Rock bands, that included Jazz elements in their music. A trait they followed all throughout the 70s. In their album Chicago XIV, they started leaning towards a more New Wave direction which didn’t last for long. In their album Chicago XVI they started to develop a more AORish sound, but still maintaining the Jazz elements of their 70s era. On the other hand Chicago XVII featured a more Hi-Tech AOR sound, which was totally new for the band, toning down the Jazzy elements by a huge degree.
5) Alice Cooper
Alice Cooper is mostly known as one of the artists that defined the Shock Rock genre. A genre that is not different musically with Hard Rock or Heavy Metal, but it mostly has to do with the live performances of those artists, which are very provocative. In the early 80s he made an attempt to follow the New Wave genre, and by the late 80s he shifted towards a Glam Metal direction, especially with Trash which is his most successful album, and also his best one along with Constrictor.
6) The Cult
In their first two albums Dreamtime and Love, The Cult belonged to the Post-Punk/Gothic Rock crowd, but since Electric they started to change drastically, leaning towards a more Hard Rock/Glam Metal sound, and the late 80s/early 90s period was definitely their peak, with Sonic Temple being their best album. Their 1994 self-titled album though, was featuring a Grunge sound unfortunately, and I believe it was their black page through the band’s history.
7) Def Leppard
Many NWOBHM bands embraced the Glam Metal sound (like Judas Priest did in Turbo, or Saxon did in Destiny), but it was Def Leppard who did it first, and also remaining in history more like a Glam Metal band than a NWOBHM band! Since their Pyromania album, they started to lean towards the Glam Metal direction and making their sound more Americanized, but it was with Hysteria that they became fully Glam Metal, something they did with Adrenalize as well. An unfortunate sound-change was with Slang also. They abandoned their good Glam Metal sound in favor of a more lousy Alternative Rock or even Poppish sound at some points.
Europe were one of the most successful Glam Metal bands of the 80s and the early 90s. Since their reunion in 2003, they changed their sound drastically, by abandoning the Glam Metal sound they were known for and following a more Modern Rock direction. Maybe it was a silly move not only quality-wise, but commercially as well. Although they are still popular in their homeland (Sweden), they are not the international superstars they were in their The Final Countdown and Out Of This World days.
During the 70s they were one of the most popular British Progressive Rock bands. But after the departure of Peter Gabriel, they started to follow a more commercial radio-friendly sound slowly-slowly, preferring short length songs, instead of longer ones unlike they did in their albums with Peter Gabriel in the line-up. Particularly, the album Invisible Touch was a drastic change for the band’s sound, one album that would be defined as Synth-Pop. Likewise We Can’t Dance, featured a more commercial early 90s Pop-Rock sound.
10) Joe Jackson
Joe Jackson was mostly known as a New Wave artist in the late 70s and early 80s. Since his album Night And Day, he started to follow a more Jazzy Pop direction, and also writing many Soundtracks. Night And Day was also his most successful album so far.
11) Jefferson Starship
Jefferson Starship was actually the natural evolution of the 60s Psychedelic Rock band Jefferson Airplane. Throughout the 70s they were a Psychedelic Rock/Progressive Rock band (or even Folk Rock sometimes), but since the album Freedom At Point Zero they started to develop the AOR sound that they would become known all through the 80s, and with their later incarnation Starship as well. Particularly, the song Jane marks the birth of their AOR sound, and it’s also considered an AOR all-time classic hit, in general.
Started as a Santana’s side project, Journey started as a Progressive Rock band featuring Jazzy elements in their first three albums. Since their album Infinity, and with the recruitment of Steve Perry, they started to develop their AOR sound they became known for, and also becoming an independent band on their own, to the point most people believe that Steve Perry was the original vocalist, overshadowing their previous work.
Mostly known as one of the pioneers of the Thrash Metal genre, especially with their debut Kill ‘Em All, known for its raw and aggressive sound. In their follow-ups Ride The Lighting and Master Of Puppets, they were still Thrash Metal but they were less aggressive than their debut. The change started to appear in the album …And Justice For All, where it would be considered more like Progressive Metal, than Thrash Metal. Also their song One, was their first commercial hit. Not much what we would call a sell-out, but still it was the start of their commercial breakthrough. Their commercial breakthrough came in 1991 with their Self-Titled album (aka Black Album). Here they abandoned any Thrash Metal (or Progressive Metal) traits and became more like Mainstream Hard Rock, a direction they would follow all throughout the 90s.
Ministry started as a Synth-Pop band in 1983 with their album With Sympathy, being one of the few American bands following this genre. In their second album Twitch, they changed their sound pretty drastically, following the obscure genre called EBM, which means Electronic Body Music. And by their third album The Land Of Rape And Honey, they started to build-up their Industrial Metal sound, that they would follow the later years, all throughout their career.
15) New Order
New Order are basically the ¾ of Joy Division. Or in other words, Joy Division without Ian Curtis. Their debut album –as New Order- Movement, was more or less the third Joy Division album. But since Power, Corruption And Lies they developed their Synth-Pop sound, they also followed in their album Low-Life, yet still having a dark mood in their music and a Joy Division feeling in general. But since Brotherhood they became lighter and poppier, featuring a more upbeat dancable sound, abandoning any Joy Divisionish traits from their music.
Santana became popular in the late 60s-early 70s as a Progressive Rock band which included Latin elements in their instrumentation. Since the late 70s their popularity started to decline, but they made a huge comeback by the end of the century, with the multiplatinum album Supernatural. All their albums since their debut in 1969 till their 1990 album Spirits Dancing In The Flesh were Progressive Rock (or Hard Rock also) featuring Latin elements. Their only exception was their album Beyond Appearances in 1985, which featured a more Synthy sound, to the point it could be marketed as a Synth-Pop album, yet still keeping some of their traditional Latin elements.
In 1992 with their album Milagro they started to build up the tradition, they would follow from Supernatural and onwards. Meaning, recruiting lots of Guest Musicians as lead vocalists. Still Milagro wasn’t as Poppy as their follow-up Supernatural was. In Supernatural, except being a huge comeback, it was a drastic change for the band’s overall sound, resembling more Latin/Pop stars of the era (like Ricky Martin or Jennifer Lopez) but still Carlos Santana’s Guitar playing, maintained some kind of identity in this album, which was a much better album than the crappy albums released during that year truth is.
17) Spandau Ballet
Spandau Ballet started as a Synth-Pop band with Journey’s To Glory, and in their second album Diamond they followed some bizarre Experimental direction blending Funk with New Wave elements or even Indian music. Since True which was their most successful album, they became the glossy Pop/Soul band they were widely known as, and remained as such till their break-up.
18. The Stranglers
When they released their first two albums Rattus Norvegicus and No More Heroes in 1977, they were an aggressive New Wave band with edgy lyrics. But since Black & White their sound started to become more mellow. Yet, all their albums till La Folie, still had some of this edginess they became known of with their first two albums. It was their seventh album Feline where they changed drastically and became like a new band.
The instrumentation was very different, featuring way more Synths than their previous albums, and the sound was more melancholic. In their follow-up album Aural Sculpture, they added Horns in their instrumentation and tried to make their sound more appealing to the American market, yet without any success. Throughout the 90s (with a different frontman this time) they also had a change in their sound again, which was more like a 90s version of their mid 80s stuff, and since 00s with a new frontman once again, they made a return to their late 70s roots.
19) Talk Talk
Their first two albums The Party's Over and It's My Life were leaning towards a commercial Synth-Pop direction, but since The Colour Of Spring they started to become more experimental. The Colour Of Spring actually was half commercial (mostly because of the big hit Life’s What You Make It), half experimental I would say…but since The Spirit Of Eden, they followed some weird Experimental direction, abandoning any commercial direction and sounding like a new band. Probably they might be considered pioneers of the Post-Rock genre, a genre that the band Radiohead brought to the masses in the early 90s.
20) Talking Heads
One could say, that almost every album of theirs, was different musically. Their first two albums Talking Heads: 77 and More Songs About Buildings And Food were more like some kind of Art Punk or something like that, whilst their album Fear Of Music was mostly edgy New Wavish. Their fourth album Remain In Light, was perhaps Post-Punk although not as Dark as the average Post-Punk album. Since Speaking In Tongues, they followed a more commercial radio-friendly direction but still they weren’t static in their style. For example Speaking In Tongues was Synth-Funk more or less whilst True Stories was more like Alternative Rock in some way.
PS: Sorry for the wrong category.