But why can they do a mere modification when I am moving to a higher fare - but not when I move to a lower fare? It's the same transaction logic, just in reverse.

Also, when I get involuntarily downgraded, they can do a reimbursement of the fare difference - which proves that accounting-wise it's possible.

And what about routing changes which lead to lower fares/taxes? I've had that several times, where I changed from xxx-FRA-JFK to a nonstop xxx-JFK. They simply refunded the difference.

So which element is it that prevents reimbursement of fare difference between two fully flexible tickets?