Honestly Bate, I don't think any of us knows specifically. All our WoW accounts were around at the time of the change to battle.net, and we did as we were told, which was basically either 1. Make a battle.net account if you haven't and/or 2. Log into said battle.net account and hit the merge option.
I personally had to make a new battle.net account and didn't have any games on it but WoW up until a few months ago when I finally got around to finding all my Blizz games and punching the CD keys in.