Have been in this situation many times. As has been said, you will be booked on next flight in FC if available. You will be booked in coach if no FC. However, I have always been given the chance to take a later flight in FC if I am willing to wait. Look at it this way. If you booked in F fare, they can't book you into FC if no seats available either. (of course F fare does give you more options with compensation and transfer to other carriers) This is why I like the fact that Delta holds back a few seats in FC until close to departure. It allows us to SDC and to IROPS into FC. If they allocated all seats at window, this would not be available.