Marcel,
the new quantity, will it be less or more than already received? More is not a problem. The PO will just be updated in Ariba and SAP, and the supplier ships the missing items in addition to the ones the buyer has already received and accounted for via GR. All that's needed is another GR for the difference.
When you reduce the quantity, what would you like to do about the items the buyer has received in excess? Ship them back? At who's expense? Discard them? Keep them, as a consignment stock? So typically, you would not want the system to allow for quantity reductions after shipment, when the risk has already moved to the buyer. Instead of becoming an issue with procurement, and the PO, the process may now require negotiations about logistics and financials (storing of excess stock/ consignment /shipping back) and possibly compensation, i.e. a credit memo for the buyer.
Cheers,
Andreas