In God's eyes we do change immediately from the "old man" (as the Bible would say) to the "new man." We are a new creation! (2 Corinthians 5:17) But becoming like Jesus is not something that's done overnight; it's a process that we live by taking one little step after another.
We can do that, can't we? We can take another little step past the first, and then another little one, and another ... And these little steps may not seem like much at the time, but when you look back you see progress — big progress! The apostle Paul called it transformation. And we're not alone in having it happen in our lives: the Holy Spirit is at work to help us in this gradual, but transformational change.
Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. And all of us, with unveiled faces, seeing the glory of the Lord as though reflected in a mirror, are being transformed into the same image from one degree of glory to another; for this comes from the Lord, the Spirit. (2 Corinthians 3:17-18 NRS)