I kind of wish they'd just go for full 6e. I have a feeling that these new core books won't sell as well as they hope. Sure it might have a boost from the hard-core market immediately. But what incentive is there for the casual players to upgrade? They said themselves that the old core PHB will still be usable along side the new one. So really as long as the DM has the new book there is no reason for the existing players to upgrade.