The mind can be changed but the sin nature cannot be changed even when the mind becomes compatible with the sin nature. Because the nature cannot be changed is why Romans 6:6 states that the old man sin nature was destroyed and the new nature replaced it. The ultimate end is: "But I see another law in my members, warring against the law of my mind, And bringing me into captivity to the law of sin which is in my members" Romans 7:23. At various times the mind ultimately succumbs to the control of the inborn sin-nature to those who were under the law as indicated by the word captivity which is bondage. The mind is held in captivity to the sin nature is the ultimate end of all who have not the "Christ in you" nature both in them. The individual can allow the mind to be compatible with the sin nature and when this happens it is known as a reprobate mind. A reprobate mind is one that does not want to retain God it's knowledge. "And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient," Romans 1:28 A reprobate mind and the sin nature resident within mankind are totally compatible with each other. Those under the law had to maintain a constant vigil to keep this from happening. This was a mind struggle of the worst case. Most of Christianity preach a gospel that is nothing more that attempt to change the nature of individual. Of course this is a futile attempt because they do not know that the sin nature will pretend to be dead or it will pretend to change etc. The truth is nature cannot be changed because but it can be destroyed or birthed. "That which is born of flesh is flesh, and that which is born of Spirit is spirit," John 3:6. The first three verses establish the point that a union exist between the soul/mind and sin-nature Analogy to the marriage in the first three verses that only death could dissolve. Romans chapter 6 describes how the born-again believer is joined unto Christ. The union between the believer in Christ is achieved through the work of the cross. The death of the sin nature (first husband) allows the believer to legally be joined to the resurrected Lord. This simply means that the former union between the soul/mind and sin-nature was dissolved through the death of Christ on the cross. Christ's death on the cross absolutely did once and forever destroy the sin-nature (old man) of Romans 6:6.