Before the Internet went out this morning I managed to get Atheren into Burning Crusade Classic. There are multiple confirmations that you want to play The Burning Crusade version, and I can't imagine anyone "accidentally choosing the wrong mode.
Choose Burning Crusade Classic above then confirm again here.
Confirm by Character (will need to do for each one) on Character Screen
Type in Your Character's Name in the box to give final confirmation you want this character in The Burning Crusade Classic.