If it's coach you need the exact class available. For First/D1 you just need a seat be available. SDC works the same on awards as paid tickets. If flying paid coach you'd need the same class to SDC or would have to pay the difference in fare to confirm, otherwise you could standby if you qualify. Now since you can't use SDC or SDS for international, you'd be required to pay the current amount to change those flights. However again that's the same as a paid ticket.