It pretty much is that cut and dried unless it came to force of arms. The relevant Supreme Court decision, 1868's Texas vs White, held that: “The union between Texas and the other states was as complete, as perpetual, and as indissoluble as the union between the original states. There was no place for reconsideration or revocation, except through revolution or through consent of the States.”
So the options are either:
a) Open rebellion, which would be swiftly quashed. I say this as a Combat Arms veteran: it would be very swiftly quashed.
b) Everyone legally and legislatively agreeing to go their own separate ways. Which absolutely is not ever going to happen.