In some cases, the upgrade is not just an equivalent but rather a significant step up. The company is offering, for instance, to take a customer's GeForce 7600 GS OC 512MB AGP card for a GeForce 9500 GT OC 1GB PCIe card. Even at what is considered the “low end” of the spectrum the card maker has options available, with BFG offering a 9600 GT with 512MB of RAM in place of a 6800 OC 128MB. It certainly wouldn't be a completely “free” upgrade – you would still have to finally decide to purchase a PCIe board and handle all the other upgrades that go along with it. Still, it's a nice incentive.
BFG does require hardware registration to take advantage of the offer, though they do not require an original proof of purchase. You can learn more about this offer here.