Well... technically Essentials does change quite a few things. Many wizard powers now do half damage on a miss, rogues can sneak attack once per turn instead of round, melee training got changed, weapon focus got split into weapon and implement focus, two weapon fighting got an update, rapier became military weapon, item rarity was introduced, etc. Some are good changes some are dubious. But this is no different than errata (or updates, or whatever you want to call it). And if you were playing with updates before, there is no reason to ignore the updates in Essentials.
Having said that, most of Essentials is new material and as such, additions.