Is there anything in the law that makes it illegal to change employees’ schedules?

Q. We run a small printing company and have an employee whom we want to move from the day shift to the swing shift. Although this employee has the most seniority, he has the least experience with the presses we run during the day. When we told the employee of our plans, he said that moving him would be illegal. Is he correct? We are worried that if we move him and he quits, it won’t be the last time we hear from him.