(b) A mutual insurer may amend its articles of incorporation by the affirmative vote of two thirds of its members present in person or by proxy at a regular or special meeting of members of which notice in writing setting forth the proposed amendment was mailed to all members at least thirty days in advance, except that no such amendment shall reduce the surplus below the amount required by this chapter for the kinds of insurance thereafter to be transacted and except that no such amendment shall be filed with or accepted by the secretary of state unless approved in writing by the commissioner.