Updating Quantity and delivery date from bapi_po_change.

Hi Guru's.

i had a question which is regarding PO updation through bapi-po_change.

my requirment is to update PO from PR.

i can able to update a po from PR for multiple line items.

i am facing the problem with single line items.

problem :

when i create single line item and change existing line item, which is performing two actions at a time

i can able to create a new line time from PR to Po , even i can able to change the existing line time partially that is it is updating only quantity but not delivery date, it is updating the delivery date which is created for new line item.

please help me out how to resolve this,

plz guide me what are the parameters which i supposed to pass.

every thing is working fine for multiple line items but not single line item.

Regards,

Raj.