When I change a ticket with Promo Code applied in the past online, my new flight also got the same % off as well. Not sure if this is a bug with the online booking tool, or it is something the phone agent can do as well. But I do think if your ticket is a refundable ticket and you cancel and get the refund, the promo code will be gone. You can try asking Customer Relation and see if they will you re-issue a new one to you.