There seems to be a bit of confusion over this, so I thought I'd clear it up as best I can to my knowledge. Of course, if anything seems wrong, feel free to correct me!
Full details: http://eu.finalfantasyxiv.com/en/product/
In-game bonuses (XIV:ARR CE): Helm of Light, Coeurl mount, Behemoth chocobo barding, Baby Behemoth minion.
Physical items (XIV:ARR CE): Exclusive box, art book, movie content disc, soundtrack, art cards, security token.
Pre-order bonuses (XIV:ARR or XIV:ARR CE): Early access, Cait Sith Doll minion, Mog Cap
In-game bonuses (XIV:1.x CE - the old game): Onion Knight Helm
If you own the original Final Fantasy XIV Collector's Edition from FFXIV 1.x (the old game) already
- You will get the in-game bonuses for the ARR Collector's Edition.
- You will NOT get the physical items that come with the ARR Collector's Edition.
- You will NOT get the in-game bonuses for pre-ordering the ARR game.
- You will retain your old in-game bonus items that you got with the original Collector's Edition (1.x - the old game).
If you buy Final Fantasy XIV: A Realm Reborn Collector's Edition when it launches
- You will get the in-game bonuses for the ARR Collector's Edition.
- You will get the physical items that come with the ARR Collector's Edition.
- You will NOT get the in-game bonuses for pre-ordering ARR game.
- You will NOT instantly get the in-game bonus items from the original Collector's Edition (1.x - the old game).
If you pre-order Final Fantasy XIV: A Realm Reborn Collector's Edition
- You will get the in-game bonuses for the ARR Collector's Edition.
- You will get the physical items that come with the ARR Collector's Edition.
- You will get the in-game bonuses for pre-ordering ARR game.
- You will NOT instantly get the in-game bonus items from the original Collector's Edition (1.x - the old game).
If you want everything...
- Pre-order the Collector's Edition for Final Fantasy XIV: A Realm Reborn.
- Already own the original copy of Final Fantasy XIV Collector's Edition (1.x - the old game).
I hope that clears things up a little.