If the Bandcamp user agreement dictates what they can or can’t do with your content, and they notify of a change and give you an opportunity to opt out, but you don’t opt out, then there’s probably something in the original sign up “electronic signature”, however it would be referred to when you agree to terms, which says that you agree to subsequent updates of terms for which you don’t choose to “opt out” of the agreement at time of notification.
Just to put it bluntly, if they send a notice that says “there will be some changes to our terms, click here to read about it”, that may be their only legal obligation is to send notification. I don’t know how much they can or can’t use other people’s content for their own monetary gain, but if your content is there and it’s not registered with ASCAP or BMI or any registering body that makes sure you get paid, then you’re one of the under-protected.
The ability to further pursue any claims against a major company in any arena usually comes down to who can afford better lawyers for a longer duration of time, and I know I can’t afford any lawyers for any duration so I, for one, would probably be screwed. It’s a bit of a nihilistic perspective, but if you don’t protect yourself no one else will. It’s also not to say they will wantonly steal your content, but Songtradr didn’t take over for Bandcamp because they want to grow the creativity of artists as a form of charity work, that much is rapidly becoming clear.