This is pretty much where the rubber will hit the road with respect to the backward compatibility of the updated rules and why, I believe, it would be a mistake for Wizards to identify the new books or think of them as a new edition. The more compatible the rules and features, the less likelihood of splitting the existing fan base in a significant and damaging way. If they sell it as an update, not a significant edition change, and the market agrees, then things should be pretty copacetic (excepting, again, the usual suspects dodging pieces of falling sky).