"Change" doesn't equal "Sell Out"
Change is great. All of my favorite artists have at least three "eras" of styles, some many more. As lineups change or new ideas come along, a band's sound should grow and evolve. Many bands that I generally like, don't get into my "top" list because all of their stuff tends to have the same formula and sound the same. These artists are sell outs IMO - "this formula works - no need to challenge ourselves, just keep doing the same thing".
It's when they stop being creative or completely abandon their core values/approaches to conform to commercial trends that it becomes a "sell out" situation. Nothing wrong with making money - who doesn't need money and I can't imagine a single part timer/hobbiest who wouldn't love to make a full time living playing music. It's when it becomes a situation when it's only about the money and not about the love of the art that it crosses the line.
And while I guess it would technically be considered "selling out" just to be in a cover band, it's a little different. Yeah, we're not creating new music, just regurgitating what others want to hear. But, you kind of have to be "in" in the first place before you can sell "out".