When can an employee's scheduled hours of work be changed?

Short Answer: At any time, unless restricted by contract, policy, or state law.

Full Answer: The Fair Labor Standards Act (FLSA) does not regulate work schedules. Employers can change an employee's scheduled hours without prior notice unless restricted by an employment contract, collective bargaining agreement, or state laws.
Some states require advance notice for schedule changes, known as predictive scheduling laws. If the change affects overtime, employers must comply with overtime pay requirements.
Salary must not be reduced based on hours worked for exempt employees. For specific rules, review your employment agreement and state regulations.

General Federal Standard: Employers have discretion over scheduling unless bound by contractual obligations or state laws.

Link to a legislative resource from trusted sources: U.S. Department of Labor - Work Hours