Unless such a change violates a written employment contract or union agreement, an employee handbook is not legally binding on an employer (further, is subject to change at the company's discretion). Basically most work relationships are "at will" which means that an employer can set the conditions of the workplace much as it sees fit (anbsent some form of actionable discrimination). Accordingly, the change that you describe is legal and failure to follow the new dress code can result in disciplinary action against you up to and including termination.