Dear,
What i understood from your query is that even you have done PGI of delivery against particular sales order, you can change quantity in sales order. Can you explain your problem in more detail, as this is systems standard behavior, that you can change order quantity even done PGI against any sales order, but you can not reduced quantity in sales order below the quantity you have done PGI against that particular sales order.
Let me correct,if i was wrong.
Thanks,
Pratik