"At one moment I might speak concerning a nation or concerning a kingdom to uproot, to pull down, or to destroy it; 8if that nation against which I have spoken turns from its evil, I will relent concerning the calamity I planned to bring on it. 9“Or at another moment I might speak concerning a nation or concerning a kingdom to build up or to plant it; 10if it does evil in My sight by not obeying My voice, then I will think better of the good with which I had promised to bless it. 11“So now then, speak to the men of Judah and against the inhabitants of Jerusalem saying, ‘Thus says the Lord, “Behold, I am fashioning calamity against you and devising a plan against you. Oh turn back, each of you from his evil way, and reform your ways and your deeds," (Jer. 18:7-11).
Interestingly enough, the open theists often refer to this section of scripture to prove that God changes His mind. But, if anything, these verses work against the open theist.
God works covenantally. In other words, He establishes covenants with people and ratifies them with signs. Covenants are packs and agreements between two or more parties. These covenants have stipulations and conditions with rewards for faithfulness and punishments for breaking the covenant. So, God relates to us in covenant terms.
God is telling us here in Jer. 18:7-11 that if a person repents from his sin, even though God was moving toward him in punishment He will stop and change His course of action. God is being covenantally faithful to His people. He is stating how He works. He is saying that He will relent of the punishment He was going to bring upon a people if they turn from their sin. In fact, God often tells them that He will punish them, which causes them to repent, whereby God then proclaims that He will not punish them. From all eternity God knew they would repent, but used His declarative threat to bring them to that place of repentance. If He did not tell them what would happen if they were to continue in sin, they wouldn't have repented.
Therefore, God is relating to people in their time frame, properly pronouncing judgment upon them for their sin, and then not carrying out the punishment if and when they repent. It is the means by which God knowingly brings people to repentance and is not a demonstration that God is "wishy washy."