I have Payment Terms(0001) defined in the CMR for a Customer.
Supposing in the Sales Order, if the Payment Terms are changed manually to 0005, can I have a delivery block for the same and have someone from the Head Office releasing it?
You can achiev this by activating the Critical field check in the credit management. If oyu activate the critical field check then when you change the payment term then the sales order can be blocked and further processing will not be possible. But ensure the settings are done in OVA8 transactions.