90% sure it will be the same client, there is an option to change expansion levels in the menus, your achievements and portraits should carry over, there are all new portraits and achievements, if you've left your launcher open for any serious amount of time after the newest WoL patch you should have a good amount of what you need to start HotS on launch day it'll all just depend on you game license level.
so given i downloaded a seriously massive patch a few days ago, and then another a few days after that (the first patch changed the loading screen to Kerrigan).
is it possible we already have it?
I cant imagine Blizzard allowing millions of Downloads on opening day, as it would severley lower the DL rate, thus angering a many lot of gamers...
Actually.
Do we get it our (AEST) time on March 12, or Americas?
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