If the PS3 version is any clue, you will get a download code you can use to temporarily play the game ahead of when you get the actual codes allowing you to play for as long as you have an account. You can still play on the other versions you maintain, you just can't go into Ishgard on them till you can those versions as well. Yes, I have every belief that, like the core game, you're going to need to buy Heavensward for every version you play.