don't know how the system handles in practice, but I beleive it's supposed to be like status - your benefits are based on the status at the time of travel. So if you don't have the card then, don't expect to keep the benefit, though it's possible the system may still honor it.

Also, this isn't really a basic economy question - should probably go in the general Explorer thread or on its own - checked bag charges/waivers have nothing to do with a basic economy fare, and are the same whether on a BE or regular fare.