There are formal mechanisms they use to market the
free dining promotion, but to be honest, because we hear about it here first, and with more detail, I've never paid attention to what the formal mechanisms were. You can just keep your eye on this forum. You'll see messages posted with the information, and the FAQ will be updated.
Whether you can switch to the free dining promotion later depends on several factors. First, you'll want to be sure that you book your reservation with someplace that eventually would offer the Dining Plan, but keep in mind that we don't know which sales channels will offer free dining this year (though I'd guess if anyone would, Disney's own reservation office surely would). Second, keep in mind that you can only switch if you're on (or allowed to upgrade to) a Magic Your Way package, that includes admission purchase.
Don't bother making a reservation now, if you won't use it if free dining isn't offered. Having a reservation doesn't help you at all in terms of getting the free dining offer.