Restrict changes to PO created frm Preq for network activity

[Jef]

Purchase requisitions are created automatically for components in network activities (Project Systems).
This company wants that it's impossible to change the component data (description, quantity, price, vendor, ... as entered in the network activity) in the PO created with reference to this purchase requisition.
When they look at the components in the network activity, they want to be sure that this equals what has been ordered.
Any ideas where to start?
How much time would it take to set it up like this?