May I ask which scripture you are considering? But, to answer your question as it stands the natural man does not change into a spiritual man. The reason is that the use of the concepts addresses ORIGIN.
The word "Natural" means that the object has its ORIGIN in NATURE. In the case of a man, since all men come from the womb, we are all NATURAL MEN. The only reasonable end to this is death because in that ORIGIN, or BIRTH, you inherited Adam's corruption. There is not way to change this. God let's it die.
Likewise, the word "Spiritual" means that the object has it's ORIGIN in the Holy Spirit. We see this in John 3:6. The human spirit is born again - not the soul and the body. God's salvation process begins with the human spirit, where God makes His home, and then, by a process of transformation, the Holy Spirit changes the SOUL. The soul of a man is made of mind, emotions and will. These are changed (e.g. Rom.12:2), so that they tend to take orders from God Who is a Spirit in your human spirit. The resulting thoughts (mind), feelings (emotions) and decisions (will) are thus SPIRITUAL. They had their ORIGIN in the human spirit led by God's Spirit. An example is 1 Corinthians 2:14; "But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned." The word "KNOW" pertains to the MIND. "Know-ledge" is a function of the mind. So the unregeneate man, the man who comes from the womb, cannot KNOW the things that have their ORIGIN in the Spirit. But a man who is born again, he who has had a new and spiritual birth, is capable of understanding the things "received" from the Spirit of God.
Then, one day your "Natural body" will die and return to the elements. At length, God will recover your body in resurrection. But this body is not resurrected via a womb. It is made by the Holy Spirit (Rom.8:11; 1st Cor.15:44; 2nd Cor.5:1-3).