Once you've upgraded your account (by buying the game online) you'll be able to unlock the game without any additional downloads (if you have the demo installed already )
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(petercollins)
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by the_angry_angel : You can't just respond 'yes' when there are two options. This was not a helpful response.
S2 is the full package of what is available. You can play with people who have S1 and even still play on demo servers (but why would you once you have S2 ).