The stock price could go up or it could go down. It depends how the markets perceive the chairman's influence on the company. It could be positive if the traders feel the chairman was not effective and needed to be replaced. It could be negative if the traders feel the chairman is indispensable.
A good example would be Steve Jobs at Apple computer. Many feel his vision would be hard or impossible to replace. When he was ill a few months ago the stock price went down.