Section 71. A corporation may, subject to the provisions of section eight, authorize, at a meeting duly called for the purpose, by vote of two-thirds of each class of stock outstanding and entitled to vote thereon or, if the articles of organization so provide, by vote of a lesser proportion but not less than a majority of each class of stock outstanding and entitled to vote thereon, any amendment of its articles of organization; provided, only, that any provision added to or changes made in its articles of organization by such amendment could have been included in, and any provision deleted thereby could have been omitted from, original articles of organization filed at the time of such meeting. For the purpose of this section, if any such amendment would adversely affect the rights of any class of stock, the vote in the proportion provided for in or pursuant to this section of such class, voting separately, shall also be necessary to authorize such amendment. Any series of a class which is adversely affected in a manner different from other series of the same class shall, together with any other series of the same class adversely affected in the same manner, be treated as a separate class under this section.