What is the authorization_status set to at header and line level..
I think there would not be a way to modify this from PDOI or UI. However you may try updating authorization_status='INCOMPLETE' in po_headers_all table for the specific PO and test.
It would probably a better idea to request for a dfix from Oracle(If feaseble)
I found out an API which allowed me to update REJECTED POs --> po_document_update_grp.update_document. After updating PO, I was able to kick off approval again using PO_SUP_CHG_REQUEST_WF_GRP.kickoffpoapproval.
This can be done through application also.
Buyer can open the Rejected PO from PO Entry form and change the values as per the requirement.
Can submit for approval again.