Ranger and monk were changed substantially; by WotC's own admission, ranger is practically a new class. Races and backgrounds are quite different as well, and many spells (and all the classes) have small changes that will come by surprise. The way the new rules codifies different types of actions is new. It's reasonable to expect that the non-mechanical discussion bits will be very changed as well, in fact I'd almost buy the books just to see how they explain the game in those areas, as it would greatly inform their still rather opaque design ethos.
In short, a change log between the two PHs alone will be enormous, let alone the other corebooks. How is this not at least 5.5?