I don't think you can upgrade to the CE edition, you can upgrade from standard to the digital deluxe version, but I think that's the only way.
You would be able to add a second serial number to your account, I would assume. Most other MMOs allow this. You would then have 2 versions of the game registered to your account and would get the extras from both on the same account.
This is exactly right. You'll simply have to buy the CE and add the key to your account.