If you already have 1.0, no need to buy and Legacy was a special bonus for those of us who stuck around and payed again when subscriptions started up again. You basically had to pay for 3 months of subscription between january and september last year. As a Legacy member we get a discount on monthly fee and our own special chocobo barding I think, oh and our names will be in the credits of the game.
As well as not needing to buy the game again, those of us who purchased the original collectors edition of XIV 1.0 will be awarded the virtual in-game bonus items from the XIV:ARR collectors edition.
That's the Helm of Light, Coeurl Mount, Behemoth Barding and Baby Behemoth Minion.
It's my understanding that anyone who purchased the original base game (and not the collectors edition) can obtain the above in-game bonus items by purchasing the XIV:ARR Collector's Edition Digital Upgrade (which I heard will cost $20 when it becomes available).