Yes, for most employees and in most states.
The Fair Labor Standards Act allows employers to change an employee's work hours without the employee's consent and without giving notice for the change. In terms of changing duties, at-will employees can have their job duties changed by their employer. There aren't any specific federal laws that dictate what kind of notice has to be given for changing duties.
State laws differ on an employer's ability to change employees' hours and duties. In Connecticut, for example, changes made to employees' hours or duties must be preempted by a written notification before the next upcoming pay period. Be sure to Google your state's labor laws to ensure compliance with your local labor laws.
If you're looking to change an employee's hours or duties, it's best to communicate the changes as soon as possible.
State Wage and Labor Laws - DOL.gov - Directory of state wage and labor laws
Furloughs, Hour Cuts, and Pay Cuts: Your Rights- NOLO.com - Article on reducing hours and pay