"Arise, go to Nineveh the great city, and cry against it, for their wickedness has come up before Me...3:2Arise, go to Nineveh the great city and proclaim to it the proclamation which I am going to tell you...4:2And he prayed to the Lord and said, “Please Lord, was not this what I said while I was still in my own country? Therefore, in order to forestall this I fled to Tarshish, for I knew that Thou art a gracious and compassionate God, slow to anger and abundant in lovingkindness, and one who relents concerning calamity," (Jonah 1:2; 3:2; 4:2).
God states in Jeremiah 18:8-9, "If that nation against which I have spoken turns from its evil, I will relent concerning the calamity I planned to bring on it. Or at another moment I might speak concerning a nation or concerning a kingdom to build up or to plant it." We see that the Lord has said He will not bring judgment if that nation turns from its sin. Nineveh did turn from its sin after Jonah gave them the warning from God.
"Then the people of Nineveh believed in God; and they called a fast and put on sackcloth from the greatest to the least of them. 6When the word reached the king of Nineveh, he arose from his throne, laid aside his robe from him, covered himself with sackcloth, and sat on the ashes. 7And he issued a proclamation and it said, "In Nineveh by the decree of the king and his nobles: Do not let man, beast, herd, or flock taste a thing. Do not let them eat or drink water. 8"But both man and beast must be covered with sackcloth; and let men call on God earnestly that each may turn from his wicked way and from the violence which is in his hands. 9"Who knows, God may turn and relent, and withdraw His burning anger so that we shall not perish?" (Jonah 3:5-9).
So, the open theists have no claim that this verse proves God changes because He discovers something new -- as in people's repentance. From all eternity, God knew they would repent. He simply ordained the means of that repentance by threatening Nineveh through the prophet Jonah. They repented and God relented.