Tenn. Code Ann. § 7-1-106

Any smaller city that has been excluded from a metropolitan government created and established under chapters 1-3 of this title may at any time surrender its municipal charter and become a part of that metropolitan government under such terms and conditions and by such methods and procedures as may be established in the charter of the metropolitan government; provided, that no such consolidation shall become effective until submitted to the registered voters residing within the smaller city and approved by a majority of those voting.