We know that Christ was foreordained before the foundation of the world, this is an unchangeable fact.
1Pe 1:20 He indeed was foreordained before the foundation of the world, but was manifest in these last times for you
Yet there are plans that change. King David as an example states:
Psa 139:16 Your eyes saw my substance, being yet unformed. And in Your book they all were written, The days fashioned for me, When as yet there were none of them.
God planned David's life. To what degree it does not state. But his life was planned. But we know that God is not responsible for David's sin.
Jas 1:13 Let no one say when he is tempted, "I am tempted by God"; for God cannot be tempted by evil, nor does He Himself tempt anyone.
So man is at least responsible for his own sins. And we know sins have consequences, story-changing consequences. A son born in fornication can live a long life, and influence many.
We see when David sinned, that God changed David's course setting into motion a corrective plan.
2Sa 12:10 Now therefore, the sword shall never depart from your house, because you have despised Me, and have taken the wife of Uriah the Hittite to be your wife.'
So although God may well have planned out the people who will exist, it does not mean these plans are static. They could change based upon man's actions. We see this in Genisis:
Gen 6:5-7 Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually. And the LORD was sorry that He had made man on the earth, and He was grieved in His heart. So the LORD said, "I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them."
We see here the LORD was sorry he had made man. You can not be sorry for something you have perfectly planned out. The LORD in seeing how bad man had become limited the lifespan of man to 120 years.
Gen 6:3 And the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years."
We see also in this scripture that God describes a "striving" or trying to change man. I believe God has an eternal plan in place but it shifts based upon how man responds to God.