How to switch back from burning crusade to original World of Warcraft on battlenet account?

See i upgraded my account last week on the original world of warcraft and i was looking thru the buring Crusade expantion pack and pressed get trial now it wont let me long on world of warcraft. It says Ur computer is not authorizedied to run this type of world of warcraft please download th buring crusade and do i lose my upgrade if i cant get the classic 1 back