(a) A municipality or an existing board of trustees may not exercise a power provided by this subchapter in relation to an obligation issued before June 14, 1989, unless the ordinance authorizing the issuance of the obligation or the deed of trust or trust indenture securing payment of the obligation specifically allows the municipality or board to exercise the power. The authority of the municipality or board in relation to that obligation is subject to any restriction or covenant contained in the ordinance, deed of trust, or trust indenture.
(b) The board of trustees may authorize, issue, and sell additional obligations on a parity with an obligation issued before June 14, 1989, if the ordinance, deed of trust, or trust indenture provides for the issuance of the obligations. The obligations must be payable from the revenue pledged to pay the previous obligation and must be secured by pledges and liens on a parity with the pledge securing the previous obligation.