No bonds shall be refunded under the Water and Sanitation District Act unless the bonds either mature or are callable for prior redemption under their terms within ten years from the date of issuance of the refunding bonds, or unless the holders of them voluntarily surrender them for exchange or payment. Provision shall be made for paying the bonds refunded within that period of time. Interest on bonds may be increased, but it may not be increased to a rate exceeding six percent a year. The principal amount of the refunding bonds may exceed the principal amount of the refunded bonds, but only to the extent that any costs incidental to the refunding bonds or any interest on the bonds refunded in arrears or about to become due within three years from the date of the refunding bonds, or both the incidental costs and interest, are capitalized with the proceeds of refunding bonds. The principal amount of the refunding bonds may also exceed the principal amount of the refunded bonds if the aggregate principal and interest costs of the refunding bonds do not exceed the 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 being refunded so long as provision is duly and sufficiently made for the payment of the refunded bonds.