Hi Sai,
You cannot modify for the released Document Info Records (DIR), As per your above info the Document Info Records already used in ECM process. The best way to store all the Original attachments is try to create new versions of those DIRs and upload the Original attachments.
I have one more work-around solution in my mind, but i have not tested. Try to change the status of those documents using this BAPI "BAPI_DOCUMENT_SETSTATUS". If this works, then try to upload Original attachments to the DIRs.
Thanks & Regards,
Prasad.