As long as the articles of incorporation (or the state nonprofit corporation law if the articles are silent on the point) do not prohibit the corporation from having members and do not require multiple members, you probably do not need to amend the articles.  (See Ready Reference Page:  “Articles of Incorporation Establish Basic Form of Nonprofit Corporations”)  You can accomplish the sole membership status by language in bylaws that so provide.  (See Ready Reference Pages: “Bylaws Function as 'Constitution' of Nonprofit Corporations” and "Sole Member Bylaws Can Protect Founder of Nonprofit") Be sure to provide that neither the articles nor the bylaws can be amended to eliminate the sole membership without the approval of the sole member so long as the sole member remains active.