Dear ,
As you mention that you are getting the Message "consider subsequent document" it means Please check all the Documents which are created in the doc flow.
After looking all those, Please follow the steps as TW Typewriter Suggest.
Cancel the Billing Doc i.e credit note G2 ( VF11 ) Please check either the period is open or closed . After that Cancel the Return Delivery ( VL09) , Delete the Delivery, And than Make the Changes in the Order.
Thanks
Dinesh