If he cannot lift that rock, he will destroy it. And when he can destroy it, he is still omnipotent and omniscient. And you just said that IF HE CAN LIFT THE ROCK,THEN THE ROCK IS NOW UNLIFTABLE. Why would the rock be unliftable? God has already lifted it. It will just make him more omnipotent and omniscient if he can lift an unliftable rock, wouldn't he?