Nice find. It does require a very special set of circumstances, but could be useful. Take advantage of it while you can!
Please do report back if you do actually receive a refund for that fare difference.
The method mentioned by @
pascalgr definitely works. I had to rebook an AF fare last month where the total fare of the original ticket plus the fare difference would be more than the fare of a new booking. Booked the new flight, and requested a refund for the old one. Received the voucher not long after.