LAbor Relations: Refusal To Work Overtime

The labor agreement stipulates that " changes in the work schedule" must be mutually agreeable who has refused to work 6 hours of overtime as he was requested to do by his supervisor, is given 1-day suspension foe his action.  The union supports Gryzmisk all the way to arbitration on the grounds that the relevant overtime constituted a " changes in the work schedule" that it had not approved.  The management argues before the arbitrator that the 6 hours in no way could be considered a change that needed union acceptable since it was for a "limited and specified" duration.

As the Arbitrator in this case, What would you have decided?