It wouldn't be as simple as that. And I don't really have any educated answer, but I imagine the board could replace him with whoever they choose. That could be themselves, or it could be someone in the same ilk of David Gill and Peter Kenyon. Not people that have shares in the club, but people that can run a football club, properly.
That's the tragic thing, Shepherd and Co don't necessarily have to leave, they only need to appoint someone to run the fucking club. They could sit back and rake in the cash, with no effort. But that leads me to believe Shepherd enjoys the limelight.