Edit: This article was sourced from Blizz. On the day of patching, you'll be able to open Battle.net desktop and choose the WOW TBC Gold game you want to play: WoW Classic or Burning Crusade Classic. Both games will show your characters that are currently playing with the old name or. When you have chosen a game choose a character that you would like to play and then confirm. You may also use an optional paid service to play the cloned character in the two game types if you choose to play as your character on both. All you need to do is sign in the game as usual and begin playing.
Since the copy has been completed prior to release Yes. Classic includes all characters, therefore there is no delay in copying if you select Classic. TBC can cause delays, simply because it's not immediate. It can cause more characters buy WOW Burning Crusade Classic Gold being copied simultaneously. This could result in the system being unable to function properly.