Update another users session data

Typically, writing to the session data, affects your own session only.
What you could do, is check the user's role in the database on every page request.
There are several ways to do that, e.g. autoload a helper function or a library. Or base all your controllers on "MY_Controller" and put the code in the constructor of MY_Controller.
If the role in the database differs from the current role in the session, reload all user permissions and update the complete session. That way, the page that is being loaded, will only have the content that this user is allowed to see.