NWA settlement rule warning message "Receiver OR..... is already in settlement hierarchy"

Hi SAP gurus

I wonder if anyone can help advise on the following warning message we are encountering when trying to update the settlement rule on a NWA.

I have attached a screenshot but when trying to create a new settlement rule with Receiver Type WBS, valid from 10 2016 onwards (delimiting the previous CTR settlement rule at 9 2016), we are getting the warning message "Receiver OR..... is already in settlement hierarchy".

This Order has been settled to the NWA so therefore was a Sender to the NWA, and we are now updating the NWA settlement rule to a WBS receiver and this Order is not a receiver in this case. Any ideas why we are getting this and what I need to do to eradicate?

Thanks Sharon