Changes in accounting from requisition to purchase order

Dear all,

In some cases I need to change one segment in the accounting flexfield for the accounts created in purchasing - example: on the requisition we still don't know the supplier so the intercompany segment is the default, but if we end up buying from a connected company I need to change the value in the intercompany segment to the appropriate one.

How can I do this? Is there a node in the PO Accounting Generator that handles the case when PO is autocreated from a requisition (cause I can't find it)? Or does the account copy happen during document creation and PO Accounting Generator never gets triggered?