God is perfect. He is entire within himself and in need of nothing. His perfections were as they were without creation, they are what they are upon creating, and they are what they would be if creation ceased to be. Therefore, God's perfections are not reliant in any respect upon creation. It could come and go but God would be the same.
When God created the universe and established something other than himself, of necessity, changes occurred within him due to his relation to that reality. That reality could have nothing to do with his perfections or else he wouldn't have been perfect apart from creation. The changes that reality caused upon its coming into existence, likewise, could have nothing to do with his perfections. So even though God does not change in regard to perfection, change is still part of his experience.
Before or apart from the decision to create, God had no relation to creation. If that reality was in his mind eternally (as would be the case on Essential Omniscience, Determinism, or Molinism) but undone, he would have been less than perfect apart from creating. He would have needed to create in order to fulfill that unfulfilled ambition, or to instantiate that knowledge in his mind which had no source in existence. Therefore, when creation did not exist God had no knowledge of it--there was nothing to know.
In his eternal perfections apart from creation, he did not know creation, created beings, nor the things that created beings did. Once he decided to create, he knew creation entirely. He knew created beings and all the things created beings did, completely, utterly, exhaustively. He did not and does not know them because he pre-conceived them in distant ages of eternity, but that in instantiating what he did conceive he knew them entirely through his omnitemporal omniscience. He knows them from the the foundations of the world, even though he wouldn't have know them "before" the foundations.
God can change in regard to thought or notion and still remain eternally perfect, as odd as that might sound. It is not at all necessary that God would have to know everything about creation before he conceived it and said, "Be." Theological conceptions which deny the first and insist upon the second lead to bizarre conclusions. The picture of God gets painted with inappropriate colors (as in Calvinism) and/or self-descriptions the Holy Spirit inspired in the Word of God get denied (as in Impassibility) as a result.
That something dawned on the mind of God (like creating), and represented a change for him, which affected him, should not be too difficult a concept to grasp given the testimony of scripture.