Hi, hope this will help.
There's 3 products:
- Battlefield 3 - normal game, without expansions
- Battlefield 3 Premium - an access pass for all expansions for your account
- Battlefield 3 Premium edition - Battlefield 3 + Battlefield 3 Premium in one
Now when you get a new game, you get a cd-key, not an account - you don't get accounts, you always have to make the accounts yourself and add the cd-key to your existing account.
So if you have Battlefield 3 and you get a Battlfield 3 Premium key you just simply add that key to your account - this is the easiest way.
If you have Battlefield 3 and get a Battlefield 3 Premium edition - it depends. If Battlefield 3 Premium Edition contains 2 keys (one for bf3, one for premium) you can simply add the Battlefield 3 Premium key to your account. If it's 1 key think it will work as well, it wont remove your stats at least so don't hurt trying. If it for some reason doesn't work and you need to contact EA at help.ea.com/contact-us
Remember to accept the solution if it's correct and press the XP button if somebody helped.
Cheers
-Carbonic