I have the same question. Anyone have first hand experience with this?

I actually went through and changed my Classic Economy to Flex Economy because the difference was smaller than what I would have to pay for seat selection for my itinerary. Then I chatted with the agent about canceling this new Flex ticket for a full refund. The agent insisted that I would get a full refund, even though I was very explicit with him that this was originally a non refundable Classic ticket that I later changed to Flex.

I am tempted to test this and cancel this newly Flex itinerary - for an additional $100 or so I can replace my itinerary with a Premium Economy (Classic). What is extremely annoying is that if I try to change my itinerary to Premium Economy online, it gives me a quote of $2590 for Premium Economy Classic instead of the $100 or so actual fare difference. And what is even more strange is that the website is giving me a smaller quote ($2000 even) for Premium Economy Flex... And the agent insists this is not a bug, it's perfectly normal... If I were to buy this itinerary out right, Premium Economy Classic fare is $1953...