I just got contacted by SE, their answer:

Thank you for contacting the Square Enix Support Centre.


We thank you for your interest in FINAL FANTASY XIV: A REALM REBORN.

You will be able to use your FINAL FANTASY XIV disc and registration key to register for an account an play FINAL FANTASY XIV: A REALM REBORN.

We trust this answers your query, however if you have any further issues, do not hesitate to get back in touch.


Kind regards,

The Square Enix Support Centre team.