So long as there are outstanding any bonds to which collections of the costs levied by this subchapter are pledged, the General Assembly may eliminate, modify, or otherwise change the costs levied by this subchapter. These changes may be made only on the condition that there is always maintained in effect and made available for the payment of outstanding bonds sources of revenue which produce revenues at least sufficient in amount to provide for the payment of the principal of and interest on the outstanding bonds and to comply with all covenants, including, without limitation, the maintenance of funds and reserves, in favor of the holders or registered owners of the outstanding bonds or the trustee for the holders or registered owners of the outstanding bonds.