You have to change your ticket. FlyNow is only available on the day of departure. When you check-in for your Monday flights on Sunday the FlyNow option will not even be available to you. You'd have to wait until midnight Monday morning.
Change fee + fare difference is the only way to go here. Unless this is the return portion of a trip - then it's usually just the change fee.
You can't even do standby on this as standby is only valid on the day of departure as well.
-RM