I know this has been answered tons, because I myself have done so numerous times, but whatever. If you have a 1.0 account, you don't need to register anything. You will be sent an email regarding where and when you can get the client. Also, you will get to play from launch until Sept. 9th for free in the Welcome Back campaign.
If you bought the 1.0 version, but didn't register the game code, you will be able to do so on or after Aug. 20th. Since it is considered a new code, you will get 30 days free time with it. It seems to be unclear whether this will be added to the 2 week Welcome Back campaign or simply take place of it. I'm opting for the latter myself, as that seems most reasonable from SE's perspective.
Finally, if you have just bought the 2.0 version, everything applies the same as if you had bought the 1.0, yet hadn't yet registered the code.