Refunding revenue bonds may be issued in the same manner as the bonds which they are to refund, in the discretion of the legislative body, upon written request of all the holders of unpaid certificates of a particular issue payable from the revenues of the waterworks system as herein provided. The amount of such refunding bonds shall not exceed and may be less than the amount of the certificates to be surrendered and shall not exceed and may be less than the determined value of so much of the system as was paid for by such issue of certificates, less the amount of the certificates paid.