Yes! I just grabbed this! I cancelled a very expensive fare and rebooked. Not only had the price dropped since I booked, but I got 50% off the lower fare.
$424 (incl taxes and fees) cancelled and rebooked at $342 when they had the 20% off promo.. cancelled and rebooked AGAIN... new fare = $302... 50% off the base fare makes my total $157.60. Not bad for F on a busy travel day!
No cancellation fees with credit card = big win.