When you upgrade an account for the Beta, you're asked to create a blizzard account. At that point you can instead log in with an existing blizzard account and tie all your WoW accounts together.

The _only_ impact this has is that you get a popup to select which wow account you want to log in (if you don't save the account name) or a dropdown if you have it save the account name. Once you log in with your account there's no difference between blizzard account and the old style login. Your accounts are still completely separate and can be logged in at the same time.

Think of it more like having a master username/password that works with all your wow accounts instead of actually linking them together.