Wait, so is it an entirely new game or a patch for the game?
It's a patch for the most part, since you need the original game in order to install YnK. In a sense, it's also a new game, since you need a new save file.
It's a new, separate game (which means you need to install it in a different folder) that requires you to have the first game installed (it will refuse to open if it can't find the folder for that game). You can import your cleared save file from the previous game, but your progress won't be kept -- if I understood the manual correctly, you'll get something similar to the save import feature from B2W2, where you get a few flashbacks and new events if you give it a save file. It also looks like you get something new for your starter (regardless of it you choose the same one or not) and you keep your boxes as well? Google Translate wasn't too clear on that matter.