Absent a contract with the employee or collective bargaining agreement (union contract) that restricts the employer's ability to do so, yes.
The employer must follow California law for payments and breaks, but may otherwise schedule workers to come and go consistent with its needs. There is a restriction on very short shifts -- in California, if the employer requires an employee to come in for a shift, the employer must pay the employee for at least half a regular shift or for four hours, whichever is less.
If the employee is unable to work the new hours due to such factors as the unavailability of child care, the employee may be eligible for unemployment if he or she has to resign due to the new schedule. That's something to investigate based upon the full facts before resigning, as resignation will normally prevent you from getting unemployment benefits, but it's also something that normally must be done before you begin work under the new schedule.

