The other day I was doing a workshop on governance issues and someone asked the question about how to navigate principal turnover, especially if the change isn't exactly cordial. This is a very tough situation, but one that governing boards must successfully maneuver on behalf of their school.
Principals typically seek new employment January through March. This means it's good to have the pricipal evaluation in January to make sure that the conversations are early enough for both parties.
Of course this results in the obvious difficult situation of the principal needing to finish out the school year with integrity. I don't know any "magic pill" to make this scenario any easier, but I do believe that everyone involved should be totally honest with each other and "take the high road" even if that's the most difficult road to take.
Part of the role of the board president is to be the primary communication link with the principal. This is where the conversation to change administrators begins. Usually things happen leading to a princpial seeking a different position and so often the discussion can be anticipated.
A principal leaving before the end of the school year is almost never good for a school. I tell charter school board members, hesitant to finish out the school year with a principal they're anxious to see leave, that unless the principal has done something illegal or they have reason to believe the principal will attempt to destroy the school, don't terminate someone early.