Here is the thing, I bought a WoW battle.net account online and changed all the information.
However I am still afraid that the original owner would take it back so I created a new account with the same last name as the original account, but the other informations are different (first name, secret question and answer etc) and I transfered all the characters onto this new account.
My question is whether the original account owner is able to get the characters back if he calls Blizz.

Plz help me!

You made it look like you hacked the account...he could say that to Blizzard.