Originally Posted by
hhdl
AF brands are mostly decoupled from fare buckets now. There are light, standard, and flex fares in most of the fare buckets, so for the most part the bucket availability is the same for light and flex. However, the light fares tend to only be filed with stricter conditions (e.g. longer advance purchase or stay requirements or perhaps even (a la what the US airlines have started bringing back, requiring accompanied travel)), so there are occasions where there's bucket availability for light fares but your trip doesn't satisfy other requirements for any other light fare.
Indeed.
But I was answering the original
Lemonwings question, where he found that changing the return date led to a higher Flex fare than newly issued Flex tickets.
My point was about the reason why changing an existing ticket, in say Flex, could naturally lead to a higher fare than lower fares available at the time of the change. My guess is because the new Flex is issued in a lower fare bucket. I doubt that the existing booking would allow to downgrade the existing fare bucket just because you change the date of one of the segments. As you (and I) mentioned, it could also be that the fare basis, say K*******, needs to be changed to another (more expensive ) K%%%%%% because some conditions are not met (advance purchase or stay requirements), but that should also apply to the new ticket.