Unless there's a contract guaranteeing a shift, the employer has broad discretion. If any employer moves a single parent, with no caregiver for kids, to nights, the employee might have a discrimination claim if they're being singled out due to gender/marital status, though some evidence of improper intent may have to be shown. A significant change in schedule might also entitle the employee to quit, and still get unemployment benefits, due to the change in work conditions. Most hospitals and healthcare facilities provide shift-differentials (more pay for nights), and under the FLSA, the entitlement to overtime in hospitals is a little different than other types of employment (using a two week period with 80 hours, rather than 40/week, due to the nature of the work). There are several good employment lawyers in Houston if you believe you have an overtime claim or some other discrimination claim related to the change.
