(a) Unless the articles of incorporation provide otherwise, if a vacancy occurs on a board of directors, including a vacancy resulting from an increase in the number of directors:

(1) the shareholders may fill the vacancy;

(2) the board of directors may fill the vacancy; or

(b) If the vacant office was held by a director elected by a voting group of shareholders, only the holders of shares of that voting group are entitled to vote to fill the vacancy if it is filled by the shareholders.