In many jurisdictions, requiring uncompensated oncall would, by itself, possibly be "constructive dismissal". PIPing without an actual performance problem is a

In many jurisdictions, requiring uncompensated oncall would, by itself, possibly be "constructive dismissal". PIPing without an actual performance problem is also constructive dismissal, and doing it because the employee is refusing a change is possibly retaliation. Realistically, in the U.S. at least, there's very little risk that the employer will get sued, but it's still shitty and will eventually bite the employer if they get into a habit of this.