Blizzard released some information confirming there will be no restarts or patches needed to get into Draenor once midnight strikes!
Quote from: Blizzard
We are expecting a smooth transition from Mists of Pandaria into Warlords of Draenor, no server restarts are planned and there will be no patch on launch night.
If you are already ingame before midnight, you do not need to restart your client or log out of the game to activate Warlords of Draenor content.
Simply keep playing and you should be able to head straight into the fight against the Iron Horde in Draenor!