We make sales order of 50 items, we ship half of the items, and customer calls please make adjustment from my sales order, from remaining items, i need to cancel this item and add this item instead.

You are using zoho, what happened, you have to delete all the packages (even if they are 30 packages) you have to delete all shipments and packages for making the changes. OMG!

Similarly, You sent a purchase order to your vendor, and your vendor just inform you these items are not available. What will you need to do then, you have to delete all the bill, payments, and edit the PO, there should be a short cut just to cancel the item which is not billed. Simple

I dont know, why zoho have restricted sales orders to be updated. you know customers always make changes in their orders and companies have to accept those changes.