You may be able to upgrade to the Digital Deluxe Edition at any time, but pre-order bonuses in Final Fantasy XIV are a one-time deal that you won’t see again after the official launch. If you haven’t already, pick up your Dawntrail pre-order bonuses before you head to Tural.
FFXIV: Dawntrail Pre-order Items & How to Get Them
Like any other in-game item purchased in the online store or redeemed through Mogstation, FFXIV Dawntrail pre-order bonuses will be sent directly to your character via Delivery Moogle. You can access your inbox via any Delivery Moogle NPC in any town, private apartment lobby, or mailbox in your private residence (provided you have one placed on your lawn).
Go from 'Letters from Friends' to 'Purchases & Rewards' and you should find a new letter with your pre-order bonuses included.
If you only registered your pre-order code before logging in, there’s a small chance that the letter hasn’t arrived yet. Item delivery is notoriously unpredictable, sometimes taking up to 24 hours to ship purchases and rewards. In that case, keep playing while you wait for the letter to arrive.