PO with PA from Purchase Requisition on GP2013R2

Hi,

I need to create a Purchase Order with Project Accounting information (Project id and Cost Category) from a Purchase Requisition on GP2013 R2.

In a Purchase Requisition DefUsr 1 we save the "Cost Category", so the purchase operator can add this info to the Purchase Order line.

But, when i tried to edit the Purchase Order line, GP advice me that I can´t change the Project Number (which is empty) if i already add an ítem to the line (obviously from the purchase requisition)

The same problem ocurr with Cost Category, when i try to change it.

Is there any configuration that is missing?