Automatic output determination for SO when changes done

Hi

I created one output type ZNSO for sales order and created access sequence and condition record for Order Type in VV11. Now, I am able get that output type ZNSO in messages automatically while creating SO (VA01) and posting an IDoc.

But, my requirement is, If I change the sales order (VA02) that should post an IDoc automatically (User doesn't want to enter output type manually).

How can I achieve this. Plz help me on this.