Canceling a PO line

Hello Gurus

Here I'm once again, seeking wisdom!

We are upgrading from 11.5.10 to R12.1.3.  In the purchasing module, there is an interface that brings in purchase orders from an extenally managed PO System.  This interface alway brings in approved POs. As for updates to any PO line, PO core tables such as PO HEADERS and PO LINES were directly updated. We recommended against it and instead suggested using Open Interface tables.  The problem with this approach is, when a PO line is cancelled or unit prices or item description changes in the source system, our interface does not know how to handle it. Open interface always creates new document. Therefore,