Originally Posted by bocastephen

I
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.

I don't know how much work is involved in programming CO's systems to handle this process but it was apparently more than just a rule change as they were legitimately concerned about the risk of abuse unless they had a way to track the original ticket restrictions.