Hi,
PO created with old pricing procedure with tax procedure where you have old VAT value with VAT condition type.
Better set deletion flag to the line item 10 and add new item 20 with same PO , now add correct VAT amount with VAT condition type.
OR
Better set deletion flag to the line item 10 in old PO and create a new PO with correct VAT amount with VAT condition type.
Regards,
Biju K