Legal Question in Employment Law in California

If the employee is at-will, then the answer is "yes." The employer can change an employee's shift at will, and include overtime in the shift. The employee's consent is not required.

There is no law that specifically says they can or they can't. What the law says is that unless you have a personal or union contract to the contrary, you are at-will and therefore can be fired for refusing the overtime, and you are free to quit if you don't want the overtime. In either case, however, you would either be a voluntary quit, or a termination for cause, both of which disqualify you for unemployment.