Is it possible to put restrictions on editing key fields in PO based on conditions

Summary:

1. Business creates PO from Requisition in 2 ways. One is based on RFQ (called as CS-comparative statement process) and another one is based on last negotiated price from Vendor (called LPP CS - Last purchase price comparative statement process).

2. As a firm policy Business do want Buyer to modify any key information like (price, qty, addition of new line etc) in Purchase order when PO is initially getting created for the first time. That is why they wants to create RFQ for both CS and LPP CS with multiple vendor or one LPP vendor respectively.

3. Business wants buyer to give access to modify anything (all the key fields like price, quantity, addition of new line etc.) when they will create change order in the PO but wants to make sure those key fields are not editable when the PO is getting created from RFQ the 1st time.