No and no. There will be a few new achievements, as listed in the patch notes. If you want to add the new ones to your score, you'll have to go to the zones in question and kill those specific NPCs, it won't give you backwards credit. You won't lose anything you've already completed. The idea is to fill in a few gaps for things that were overlooked in previous achievement setup. This isn't the first set of such updates, and probably won't be the last.