Originally Posted by
bocastephen
The sticking point with CO was the risk of abuse. At DL, apparently their system is setup to retain the original restrictions on a ticket - so you can't book a L fare, rebook it into full Y, then make unlimited changes or get a refund. Their system will assess the change fee if you try to redo the dates or flights, and will only refund the ticket value above the original L fare (less the change fee) if you downgrade.
If they only allowed this within the first 24 hours, they wouldn't have to worry about abuse.
You can refund any ticket in 24 hours anyway, so if you're upgrading a < 24 hour old non-refundable to a refundable, you could allow the customer to have full refundability (it would essentially be the same as if the customer had refunded the original and bought the refundable ticket in the first place, so the airline shouldn't really care).