Thought of another: let’s say you have a multi-segment ticket. You want to change segment A but keep segment B. The price of new A is the same as old A, but price of B has gone way up. With option #1, you pay nothing. With #2 you have to pay the new, more expensive fare for B.
I could see #1 beneficial for someone whose itinerary is set in stone and wants to reprice the itinerary for a negative fare differential (refund) without canceling and rebooking at the risk of losing his seat/seat selection. In essence, it would be a change only to the fare component of the original itinerary with a refund of the fare differential already paid, much like the old price gurantee. So as not to allow subsequent repricing, AS has added the provision that only allows one change in that regard.
James