Cal. Water Code § 25116

If the modification is approved in the manner provided in this article, the district need not issue new refunding bonds and coupons in exchange for outstanding bonds and coupons the terms of which have been modified, but in lieu thereof the district may provide for the indorsement on the outstanding refunding bonds and coupons affected of the terms of the modification or a reference to its terms if contained in an instrument of modification or modified refunding plan, all as provided in the modification.