Corporations Code § 12536

(a) Any amendment to the agreement may be adopted and the agreement so amended may be approved by the board and, if it changes any of the principal terms of the agreement, by the members, as required by Section 12533 of any constituent corporation in the same manner as the original agreement.

(b) If the agreement so amended is approved as provided in subdivision (a), the agreement so amended shall then constitute the agreement of merger.