The board does not change the shareholder agreement nor does the board change anything else to do with the shares. What the board can do - and will do - is fire the CEO if they are of the opinion the CEO is not functioning well, but if the CEO is also a shareholder and has majority control of the stock (or the confidence of the majority of the shareholders) he/she can call a shareholder meeting, get rid of the board and re-instate himself or herself.

Boards are not active at the level of shareholders, though many boards do have major shareholders as board members.

The board can change the bylaws, issue new shares to get a majority of each class, wash out founders with a forward and reverse split. Preferred shares have all kinds of rights not given to founders. 50.1% of shares outstanding means nothing by itself.