No bonds may be refunded hereunder unless the holders thereof voluntarily surrender them for exchange or payment, or unless they either mature or are callable for prior redemption under their terms within ten years from the date of issuance of the refunding bonds. Provision shall be made for paying the bonds within said period of time. No maturity of any bonds refunded may be extended over fifteen years, nor may any interest thereon be increased to any rate exceeding six percent per annum. The principal amount of the refunding bonds may exceed the principal amount of the refunded bonds if the aggregate principal and interest costs of the refunding bonds do not exceed such unaccrued costs of the bonds refunded. The principal amount of the refunding bonds may also be less than or the same as the principal amount of the bonds refunded so long as provision is duly and sufficiently made for their payment.