Some would simply call it an evolution of style, and others might call it a sellout. I'd like to hear your examples of musical acts that changed so radically over time that you'd consider it a sellout.
For something a bit more modern, Fall Out Boy. From Under the Cork Tree was their last decent album imo and even that was a shift from Grand Theft Autumn. Hearing them now they sound like generic pop nothing to my ears. Such a shame.