The po number on invoice header can't be updated after inputing and saving

Hi Community ,
We would like to match invoice to all shipments on mutiple po and we  will use Quick Match to compelet it .
When we input one invoice to match a po number on invoice header with Quick Match button , then we save the invoice .
Now we would like to update the po number on invoice header in the same invoice, it can't be updated .
Is it stardard function for Oracle ERP R12 ? Thank you in advance