Hi Jürgen,
There is no way to modify the expense account directly on the PO line basis.
That is also not a best practice allowing users making such a change.
Even a modification of the ledger account in the accounting distribution form should be an exception and only be allowed for specific 'super users'.
If this was handled differently in AX2009, you might have to setup more items and introduce some changes in the way how PO's are processed.
Best regards,
Ludwig