The starting point is Isaiah 40:13 – "Who has understood the mind of the LORD, or instructed him as his counselor?" Rhetorical, obviously. Language is inadequate here, so when we describe the mind of God, we have to decide which language is the most helpful to describe the self-evident truth that God is omniscient.
A few chapters later, Isaiah 46 – "I make known the end from the beginning, from ancient times, what is still to come. I say: My purpose will stand, and I will do all that I please" supports this, as does the divine monologue at the end of Job – he knows all, past, present, future, insignificant, significant.
Any flavor of Open Theism that attempts to limit God's knowledge is heresy. I confidently draw the line there because it's simply against the Bible. He knew the day his Son would be presented as king in Jerusalem (Daniel 9) as well as he knows when mountain goats are born (Job 39:1) and when sparrows die (Matthew 10).
The question assumes a false premise, as well, but I'll assume it for the sake of argument. God cannot actually change his mind in any way that can be meaningfully expressed in our language. We make decisions as points in a timeline. God exists outside time. It helps our understanding to say, for example, that God is "working," and we see that as chronological. But the reality is that God is constantly in a state of work and constantly at a state of rest. Consider this 'contradiction':
John 5:17 – "Jesus said to them, 'My Father is always at his work to this very day, and I, too, am working.'"
Hebrews 4:3-4 – "And yet his work has been finished since the creation of the world. For somewhere he has spoken about the seventh day in these words: 'And on the seventh day God rested from all his work.'"
Both are true, but not contradictory. The work of God is subservient to the rest of God, because the former exists in time, whereas the latter is eternal. God 'works' in days, but he has been resting since the beginning.
The implication is that when faced with an apparent contradiction in the Word, like "The Lord changed his mind" (Amos 7:6) and "I the Lord do not change" (Malachi 3:6), interpret the more complex Scripture in light of the less complex Scripture.
Amos 7:6 adds an extra element, the mind of God. If we can assume that Malachi 3:6 is true, and that it applies to the Lord's whole being, since it simply says "I the Lord" – his nature, his attributes, his emotions, his will, his thoughts, everything – then we know that nothing of the Lord really changes, in the objective sense.
Subjectively, however, we see the actions of God in relation to ours, and since we see our actions in time, we assume that God's must be in time as well.
Revelation 13:8, depending on how you translate it, says either that Jesus was slain from the foundation of the world, or that the names of the elect were written in the book of life from the foundation of the world. Either way, you have redemption as existing outside time, and the death of Jesus and the writing of the elect are one and the same redemptive act.
And so the Son was slain and risen and the elect were chosen and purchased before any existence existed, and these were in the changeless mind of God.
Summing up quickly: Since God does not change, no part of him changes. Since no part of him changes, his mind does not change.
An argument here would be, maybe God only predetermines the significant events, and not the insignificant ones. Maybe he allows creativity, human action and chance into the equation, and then reacts to them, or guides them, or sculpts them – whatever language you like. God works "with" creation.
This assumes that there are significant events and insignificant events – a dangerous assumption, first of all. Certainly tripping on the sidewalk doesn't have the implications that the crucifixion of Christ has. But significance is relative, and Revelation 13:8, unless we are egomaniacs, juxtaposes the infinitely significant with the infinitely insignificant. Our lives are nothing to be compared with the death of God the Son. Even our accumulated lives are worthless – in Isaiah 40 God considers all the nations as "dust on the scales." We're not in the equation. The only reason we have weight is because God is sitting on both sides of the balance.
So the insignificant immediately becomes significant with the atonement, in the way that an ant on top of a skyscraper is "tall." We must assume, then, that any part of the universe considered by God to be significant is significant. Matthew 10 uses the sparrow to show that God does view everything, from quarks to quasars, as significant.
Because every event in the universe is significant to God, there are no "significant" and "insignificant" events. Only significant events and more significant events. And if the event of Christ's death was the most significant event in a gradation of significance, you can argue from greater to lesser – that God doesn't only step in on the big ones. He's intimately involved in every event.
And therefore, if Christ's death exists outside of time in the mind of God, every event does as well.
God has predetermined every event in the universe, since every event has significance and will be judged. The crack in the sidewalk that you tripped on will be judged. The lion that ate the gazelle will be judged. Creation has been subjected to frustration, and it will be destroyed.
The night that I spent on Facebook when the Holy Spirit wanted me on my knees will be one less coin I can give back to Jesus in love and gratitude when I see him. It's all significant.
What, then, of passages like Amos 7:6?
Open theism has no shortage of biblical 'proof.' God is constantly said to change his mind in the Old Testament. Even the Mosaic covenant is conditional – if you do this, this will happen. If not, this will happen.
It seems as if there are alternate futures, or that God reacts to human events. But this is our viewpoint, like saying that we stand still while the sun moves across the sky. The reality is the opposite.
When God gave the Law, he gave two conditions, blessing and curse. These aren't to reveal separate possible realities, but to explain the consequences of Israel's actions. God's predetermined course for Israel was exactly what history revealed – the rejection of God as king, the establishment of the monarchy, the division of the kingdom, the exile, the return, the domination of the Gentiles, Messiah, the destruction of Jerusalem.
In time, this looks like God changing his course, changing his mind with regard to Israel. As if his first intention were to prosper Israel in the land, but then the golden calf happened, and God got outraged and wanted to wipe out Israel, and would have were it not for the intervention of Moses.
From eternity, we see all these things as having already been completed, as having yet to be completed, as being completed currently, all these simultaneously… words fail to describe the view. But God's certainly not changing his mind based on the machinations of created beings in created time.
Instead, God planned the day that the Israelites would build the calf. He put it on the very day they were to receive the law, to illustrate the law's power to reveal sin and impotence to conquer it. He put restless emotions in the camp. He put cowardice in Aaron. He held the molecules of Aaron's fingers together as Aaron melted down their earrings. He held the molecules of the Levites' swords together as they ran them through all the people. He directed microbes in destroying the internal organs of the Israelites. Where was God not in Exodus 33? Where is he not, ever?
How can we say that we influence God, when his mind plans every one of our days to the atom? How can we say that he reacts to us, when every action of ours requires neurons, amino acids, oxygen, blood, gravity, atmospheric pressure, electromagnetism, and the weak and strong nuclear forces among millions of other life-sustaining gifts?
He sustains all things (Hebrews 1:3). In him we live and move and have our being. (Acts 17:28)