But the larger issue is to not dick over your friends or associates. I can't stress this enough. The law may be on your side, but really that is only a small amount of the total equation. I don't believe in karma (except here - upvote the shit out of me), but it will just be easier in the long run to be nice and friendly about it, even if you never see them again.
See what they think. Tell them your concerns and you're thinking about jumping ship and getting a new team together to execute the idea. If they aren't that committed to the idea in the first place, they may likely accept it and move on. Otherwise maybe you could offer them a buyout as a gesture of good faith.
I do want to make it abundantly clear that I am not from the please everybody at any cost mindset. It is just important to not dick people over when you can likely easily avoid it with words.