Cal. Water Code § 35950.1

After the board has adopted a plan of works it may thereafter by resolution change or supplement the plan if it determines that the works described in the plan as changed or supplemented will benefit all the territory within the district or improvement district, including any territory annexed thereto since the adoption of the original plan of works, and may revise its estimate of the amount of money it is necessary to raise for that purpose. Any such changes or supplements shall be accomplished in the manner hereinafter provided.