Update Purchase Order Failed (GP 2010 Web Services )

Hello , 

I am using Microsoft Dynamics GP 2010 Web Services to create , update and delete GP Purchase Orders.  

When trying to call web service method  UpdatePurchaseOrder to update a purchase order quantities (his status is ChangeOrder  and Approved ) ,   a validation exception occurred  :  

EConnectError :-9223 - Invalid PO status. The line item cannot be changed back to new. 

at :

 Microsoft.Dynamics.GP.EConnectAdapter.ExecuteWrite(String xml, String typeName, Context context, BusinessEvent eventType, BusinessObjectEventArgs args, EventLocation eventLocation)
at Microsoft.Dynamics.GP.EConnectAdapter.Update(BusinessObject businessObject, BusinessObject originalBusinessObject, Context context, Policy policy)

is it forbidden to change purchase order lines(quantities , unit costs ,..etc )  from GP 2010 Web Services or even eConnect when the Status of purchase order is ChangeOrder or approved (although i can manually do that using the Purchase Order Entry form in GP ) ?   and how can i fix it ?  

thanks ... 

Regards , 

 Hussam