Update a Work Relationship which starts in the future via workers API

Hi experts,

I'm trying to update a work relationship which starts in the future via
PATCH /hcmRestApi/resources/11.13.18.05/workers/{workersUniqID}/child/workRelationships/{PeriodOfServiceId}

For active workRelationships, I'm able to update required data as reported below:

for future work relationships (not yet started), I'm getting HTTP 403.

Any idea on which could be the reason for getting HTTP 403 status?

Regards,

Massimiliano