These are Blizzard words:
“If you haven’t made a choice for any of your character since the last time you logged in to either WoW Classic or Burning Crusade Classic, those characters will be moved to Burning Crusade Classic. The Classic Era version of the character will be permanently deleted and unavailable for cloning.”
You can easily find this with a quick Google search.
Still, if you decide to start over in Era it is a great time for it, with a TON of levelers.
I know is not much of a thing to say, just my 2 cents mate.