Description
When user tries to reschedule a meeting in which the contact or other attendees has already accepted the invite, then salesforce's standard scheduler flow core action "Save Appointment" is throwing an exception with error message as "You cannot update the response status back to NEW for an existing invitee."
Resolution
This is the expected behavior. In Salesforce, once an invitee's response status to an event has been set to "Accepted" or "Declined," it cannot be reverted back to "New." The "New" status indicates that the invitee has received the invitation but has not yet responded. This status is the default value and is automatically set when the invitation is initially sent. Therefore, updating the response status back to "New" for an existing invitee is not supported.
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See: https://help.salesforce.com/s/articleView?id=000395403&type=1