Replace the ability of the current Cabal Shadow Priest with the shadowform ability.
Note: The shadowform stays even if the Shadow Priest minion dies.
The shadowform needs some sort of stick to function and not make a uselessly slow card, but anything cheaper than 5/6 mana seems like it would be a bit too powerful.
Maybe even make it 7/8 mana and give it the 3 damage version instead of the 2.
the Cabal Shadow Priest has never really been that strong. It's usually only able to grab the late comer harvest golem if it's lucky, or it sits in hand most of the game. That, combined with the inherent problems of the shadowform spell, I say rework the Shadow Priest, and remove the spell from the game.