Hi
If the SO (from a contract) is created, and you change the contract, the subsequent document is not updated (as in other flows in SAP, as MM, PP or others)
Perhaps you can redetermine pricing in copy rules (tcode VTAA). See SAP Note 24832 - Pricing rules / TVCPF for further information.
Please, let me know if you think I don't understand you
I hope this helps you
Regards
Eduardo