Passing the "okToRehire" field in an EmpEmployment upsert request returns an ERROR with the following message:
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"okToRehire" is a field that's associated to an employee's termination, therefore it is available as read-only in the EmpEmployment entity and cannot be upserted.
If you wish to change this field (and other information related to termination), you must perform an upsert to the EmpEmploymentTermination entity.
Please refer to this page from the Employee Central OData API Reference Guide: EmpEmploymentTermination