My understanding is that yes, you would be able to register and theoretically have access to the content as soon as you got the codes. Keep in mind though, the only "content" in either expansion that you can access before level 50 is the Au Ra race. At 50 the SAM and RDM jobs become available. The rest basically requires you to catch up on the storyline(or spend even more for a skip potion). So if you arent at or close to level 50 yet there isnt a massive rush.