I had purchased a non-refundable coach ticket and needed to change it to a flight that only had 1st class - no choice, so I bit the bullet. I was surprised to find that I was not charged the usual $125 change fee. I called AS to ask and they explained that if I changed to a P fare, the fee would have been charged but if I change to an F fare, the change fee is waived.

The agent told me that, as a result of this policy, there are cases where it is actually less to change to F than to P.

Probably most of the pros here already know this, but it was a new one for me.