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 outstanding shares (Section 152) (if required by Chapter 12) of any constituent corporation in the same manner as the original agreement. If the agreement so amended is approved by the board and the outstanding shares (if required) of each of the corporations, the agreement so amended shall then constitute the agreement of merger.