I booked a flight with miles for this Friday and now am not able to travel, but I want to take this same exact flight at a later date, over 21 days from now. However there is a chance I still may not be able to fly in the 3 weeks from now. As I understand it there is no fee if you make a change and the flight date is over 21 days from making the change. So my question is how many times can I change it if in 3 weeks I want to change again and push it forward another 21 days?