
It doesn't.
Before deadline, free reissues. Why? Keeps you from just refunding the ticket and reissuing it which is what you could do if they didn't reissue it for free.
After deadline but before departure, $100 charge for a routing change. They figure at that point they can stick it to you (you can guess where) since if you have to leave by a certain date then you have no choice but to reissue it and pay the fee.
After departure, same as ONE. $75 for routing change.
In all cases, unlimited date and time changes free of charge.