As indicated by Alitalia Ticket Change Policy you can roll out free improvements inside 24 hours of bookings. If you passed up a great opportunity the free change window then you would have to pay change charge. Appointments should be made basically 7days from the takeoff. Assuming that the past flight ticket is less expensive than the new flight then pessengers need to pay the sum for changes. Pessengers are free to make changes inside the given time or after the time only one thing will be changes after the given time i.e you are charges by some fee.