Immutability

First, in Reformed theology it is said that God is unchanging in his character, will, and covenant promises. Louis Berkhof's systematic theology text (a Reformed classic) defines God's immutability as "that perfection of God by which He does not change in His being, perfections, purposes, or promises." The Westminster Shorter Catechism says, "God is a spirit, whose being, wisdom power, holiness, justice, goodness, and truth are infinite, eternal, and unchangeable." Those things do not change. A number of Scriptures attest to this idea (e.g. Num. 23:19; 1 Sam. 15:29; Ps. 102:26; Mal. 3:6; 2 Tim. 2:13; Heb. 6:17-18; Jam. 1:17).

Let's look at Malachi 3:6 in context, since that one was mentioned in the question. In chapter 3, Malachi is announcing the Lord's judgment upon his people because of their evil. But then in verse 6 he says, "I the Lord do not change. So you, O descendants of Jacob, are not destroyed." Why, though they will be punished, are they not destroyed? Because the Lord does not change. What's the connection? The answer is that the Lord does not change in regard to his covenant promises. He promised always to remain faithful to Israel, even if only to a remnant. We see this in view in the next verse when the Lord refers to the people as "descendants of Jacob." God made his promise to Jacob, and in regard to that promise he would not change. So, in the next verse, even though they have continually turned away from the Lord, he still says, "Return to me, and I will return to you." This is an instance of God remaining unchanging only in regard to his covenant promises.

So in what ways does God "change"? Simply put, God genuinely relates to people. He may be immutable, but he is not immobile. He listens to prayer; he sees our actions; he knows our hearts; and he responds. One great text which clearly explains this is Jeremiah 18:7-10 where God says through the prophet:

"If at some time I announce that a nation or kingdom will be uprooted, torn down and destroyed, and if that nation I warned repents of its evil, then I will relent and not inflict on it the disaster I had planned. And if at another time I announce that a nation or kingdom is to be built up and planted, and if it does evil in my sight and does not obey me, then I will reconsider the good I had intended to do for it."

Yes, God's eternal decree is being worked out, but the working out of that eternal decree is done through his interaction with people, and through their decisions and actions. The role that man's actions take in the working out of God's will is called a "secondary cause." The Westminster Confession of Faith explains it this way:

"God is the first cause, and in relationship to Him everything happens unchangeably and infallibly. However, by this same providence, He orders things to happen according to secondary causes. As a result of these secondary causes, some things must inevitably happen; others may or may not happen, depending on the voluntary intentions of the agents involved; and some things do not have to happen but may, depending on other conditions (WCF 5.2; I've quoted from Summertown Text's modern version).

Do you see what is being said there? From God's eternal perspective, everything is unchangeably ordered. But from our perspective, some things turn out according to our intentions. And it is our voluntary intentions which God sometimes uses as the means (secondary cause) through which he carries out his eternal decree. Kind of mysterious, isn't it? If you think so, then we're on the same page.

So let's go now to Amos 7. I would not say that this is simply metaphorical language. I believe the Lord was really showing Amos a possible future for Israel. Had Amos not interceded, we must assume God would have carried out his threats. It was in response to Amos' prayer of intercession that God relented (vv. 3,6). The third time, instead of a picture of total destruction, God shows Amos a picture of judgment (Israel is being judged, like a plumb line is used to measure how straight a wall is). This time Amos does not intercede (I assume he thinks he has no right to protest if Israel is getting a just, measured punishment for her sins).

Some may say that God was just playing with Amos, since he knew how it would all turn out anyway. Of course God did know; he knows everything. But that still does not change the fact that the way in which those events came about was through God threatening a very real punishment, Amos offering a very real intercession, and God relenting in response to prayer. It is no game or mere metaphor - God is really working out his plan through our voluntary actions. God is not changing his mind in some capricious way, he is genuinely relating and responding to his people.