Nope. You don't get a choice in the matter. Once in; in for the duration. Besides; then you'd have to root for the Longhorns.
Actually, as I understand it, there's no provision in either the US or Texas Constitutions for or against secession. Texas v. White was not based on any legal precedent, and that decision merely directed President Grant to sign a declaration re-admitting Texas to the Union. If Texas were to try to secede, the outcome would probably depend largely on the size of the cojones of whomever was in the White House at that time.