Absent a Union contract, or an agreement that runs to the benefit of the employees (such as an employment agreement) the answer is Yes. Employers never put it to a vote of employees, and while they are compelled to act in accordance with ERISA and other laws, there is no obligation to stay with the same insurance company. As a matter of good labor relations most do try to give as much advance notice as possible, but sometimes the employer may be negotiating among carriers to the last moment and doesn't have time.