(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 (Section 5034) or other person or persons, as required by Section 6012, 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.