Is God completely unchanging in God's dealing with humankind? The scriptural evidence about incidents of prophetic intercession suggests this is not always the case.
For just one example to the contrary, consider a couple of divine-prophetic dialogs. One is the dialog with Abraham, where Abraham held out for mercy against the people of Sodom. http://bible.oremus.org/?ql=147474699%20 (NRSV). In this example Abraham prevailed in the dialog but the outcome did not change.
For another example, here Moses recounts his dialog. (NRSV, Deut 9:11-21)
At the end of forty days and forty nights the LORD gave me the two
stone tablets, the tablets of the covenant. Then the LORD said to me,
"Get up, go down quickly from here, for your people whom you have
brought from Egypt have acted corruptly. They have been quick to turn
from the way that I commanded them; they have cast an image for
Furthermore the LORD said to me, "I have seen that this people is
indeed a stubborn people. Let me alone that I may destroy them and
blot out their name from under heaven; and I will make of you a nation
mightier and more numerous than they."
So I turned and went down from the mountain, while the mountain was
ablaze; the two tablets of the covenant were in my two hands. Then I
saw that you had indeed sinned against the LORD your God, by casting
for yourselves an image of a calf; you had been quick to turn from the
way that the LORD had commanded you. So I took hold of the two tablets
and flung them from my two hands, smashing them before your eyes.
Then I lay prostrate before the LORD as before, forty days and forty
nights; I neither ate bread nor drank water, because of all the sin
you had committed, provoking the LORD by doing what was evil in his
sight. For I was afraid that the anger that the LORD bore against you
was so fierce that he would destroy you. But the LORD listened to me
that time also.
The LORD was so angry with Aaron that he was ready to destroy him, but
I interceded also on behalf of Aaron at that same time. Then I took
the sinful thing you had made, the calf, and burned it with fire and
crushed it, grinding it thoroughly, until it was reduced to dust; and
I threw the dust of it into the stream that runs down the mountain.
So here is recounted a change of the divine mind in response to prophetic intercession.
It has to be said, "don't try this at home, kids." Moses was willing to place his own favor with God on the line to get God to relent against the Hebrew people.
My point: every time I think I've studied enough to make simple statements about the mind of God, something surprises me and brings me up short.