I'm not one of those people who believes that a singer or band sold out just because they changed their style.
For me, it's when an actor/writer/director/musician does something that is clearly hypocritical and/or take gigs that are largely motivated by money. I still consider it selling out even if they use "paycheck gigs" to help fund their passion projects (though to a lesser extent). The only true exception is if they're starving or in serious debt.
Changing styles drastically to me can be a sign of sell out especially if the artist started off doing something more extreme or different. It can look like a money grab as it usually coincides with achieving a certain amount of success with their original style and then they realize they can make even more money.
Metallica is a good example of this they were very heavy, very fast and then they make the black album which completely different and lighter more radio friendly. Their name is also Metallica so that lends itself to heavy music. A bit like Kentucky Fried Chicken deciding it wants to sell pizzas.
I don't follow their careers closely, but I'd probably put Nicki Minaj and Lady Gaga in that category. I've heard some of their early demos and underground releases which were much more creative and thoughtful compared with their famous pop tunes and hyper-dramatic videos.
I don't follow either but from what I know of Lady Gaga is she actually prefers heavier music which would suit her more outlandish style. But she also strikes me as someone who just wants fame and money and plays the game. If Polka music happened to be the popular thing than she would be doing that instead.
I accept the definition that would describe a person in any business who ignores their values in order to get rich, especially when they already have security and a healthy income.
WHATEVER IT IS YOU DO AND THE WAY YOU DO IT THAT GETS YOU FAMOUS IS YOUR STYLE...SUDDENLY CHANGING STYLES BECAUSE YOURE ALREADY FAMOUS/RICH IS SELLING OUT...GWEN STEFANI SOLD OUT HARD.