RE: Trying to open a Sales Order under a Purchase Order
This is standard Drop Shipment Functionality.
As you are aware, in drop shipment functionality both Purchase Order & Sales Order are linked and hence it is not ideal that you change the Sales Order, as soon as you create the Purchase Order.
This would have some implication and hence better you take cautious call and edit the sales order from Sales Order screen only.
Also, you might have different user for both the operations and hence it would not be ideal to change these details after PO creation.
If at all needed, you need to do following changes.
Page 54 > Purchase Order SubForm > Function OpenSpecOrderSalesOrderForm
Comment following line.
SalesOrder.EDITABLE := FALSE;
I am sure, you would have development access to do this modification in development area.