In Sanskrit this is called Parinam Abhava, the transformation theory. So God projects this universe, it creates it out of his, her, its own being. A changeful God is subject to change and decay and eventual death. And if you just say God has been transformed into this universe, then it's not God anymore. It's just the universe. That's the problem with this theory called pantheism.
Swami Sarvapriyananda teaches Chapter 10 Verses 4-6 of the Bhagavad Gita. In this series of talks, Swami Sarvapriyananda unfolds the highest truths of Vedanta through the study of the Bhagavad Gita (The Song of God).