(a) At any time before the expiration of an agreement governed by this subchapter, the agreement may be modified by the parties to the agreement to include other provisions that could have been included in the original agreement or to delete provisions that were not necessary to the original agreement. The modification must be made by the same procedure by which the original agreement was approved and executed. The original agreement may not be modified to extend beyond 30 years from the date of the original agreement.
(b) An agreement governed by this subchapter may be terminated by the mutual consent of the parties in the same manner that the agreement was approved and executed.