Originally Posted by
dham
It's available to all, but non-Premier members have to pay a fare difference I believe, even within the same fare bucket (if applicable). For Premier members, if the booked fare class is open on a different flight within 24 hours, it's completely free.
i always understood that if same fare bucket is available, there is no fare difference. At least that’s the way it has always been for regular changes - if there’s inventory in same fare bucket, than no fare difference, if you need to go up a fare bucket, then you do pay the difference.