The mayor and city council of any city or the chairperson and village board of trustees of any village of the State of Nebraska, which has issued valid pledge warrants, revenue bonds, revenue notes, or revenue debentures, which instruments are outstanding and unpaid, may take up and pay off any such outstanding instruments whenever the same can be done by lawful means by the issue and sale, or the issue and exchange therefor, of other pledge warrants, revenue bonds, revenue notes, or revenue debentures. Such instruments shall not be general obligations of such city or village. Any city or village which has issued and has outstanding valid pledge warrants, revenue bonds, revenue notes, or revenue debentures which are unpaid, some of which are secured by the pledge of the revenue and earnings of one public utility and others are secured by the pledge of the revenue and earnings of another public utility, may take up and pay off all such outstanding instruments by the issuance and sale of its combined revenue bonds or revenue notes which may be secured by the pledge of the revenue and earnings of any two or more of such public utilities. Any city or village may enter into such a contract or contracts in connection with such instruments as may be proper and necessary.