With a Flex ticket, it looks like you get a refund when you rebook on a cheaper flight as long as the booking class is at least the same, or higher. This means if you book Q class HEL MAD and change the ticket to HEL ARN, you should get the difference in fare back, however you can only get HEL ARN in Q class or higher, and not on O or Z class which are lower booking class, that's how I understand it. I got a refund in such circumstances a few weeks ago. if you change the ticket to a higher fare, you can downgrade the booking class. And if you pay a fare difference, it is refunded when you refund the ticket.