Well if it says "When your hero becomes Kel'thuzad", remember when a card uses its own name in its text, it's saying "this card".
So, "When your hero becomes Kel'thuzad" is saying "When your hero becomes this card". So it would trigger when you played a 2nd copy, because its a new card.
But again, we have to wait to see the actual text, see the FAQ, and any CR updates. Too early to say anything for sure.