Grants made to owners of existing franchises to operate and maintain public utilities in addition to such existing franchises of the same kind, shall be made to expire simultaneously with such existing franchises, and every such extension shall be considered a part of the property of the utility, shall be operated as such and shall be subject to all the provisions of the original grant, including the right of the City to acquire the property of the utility by purchase or lease or by any of the means prescribed by this Charter or by law.