You can install Wrath of the Lich King even before the official launch of the game, as well as upgrade your account to a Wrath account. Even after upgrading, your account will still be tagged as a Burning Crusade account. That should change by midnight (or now, if you live in Europe). You'll still be able to play the game normally, but until Blizzard decides to open up Northrend, at least in the US, it'll just be like playing The Burning Crusade. The installation features the story of Arthas Menethil, presumably to work up players into wanting to go to Northrend and kick some Lich King butt. Because, you know, Arthas is kind of a jerk.