I did not buy the new expansion pack, BUT I have used steam for a long time, and my current version of ffxi is through steam.

Steam has a default option to automatically download uninstalled games. It won't recognize another version of FFXI that didn't come through steam.

For those buying it through steam make sure you have that option turned off before buying it. once you do buy it double check in your list of games that it isn't actually d/l the game.