it looks like you have ARM. this process in NS needs to replicate what SFDC has. CPQ must drive this process, and NS - just like you said - merely reflects it. If there is an amendment (not a 50% discount :)) then NS process really depends on what exactly the amendments says (full cancellation and replacement, partial cancellation, extension, quantity change, etc., etc., etc.) this is called contract modification, btw and is entirely driven by SFDC