“At one moment I might speak concerning a nation or concerning a kingdom to uproot, to pull down, or to destroy it; if that nation against which I have spoken turns from its evil, I will relent concerning the calamity I planned to bring on it.” (Jer. 18:7-8)
First of all, “planned” could mean “devised”, as in verse 11, “I am fashioning calamity”:
JER 18:11 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.”
Also, if “devising a plan” is taken literally in verse 11 (we have to be consistent!) then don’t we have to say that God is still forming his plan? i.e. that in the illustration given here, God isn’t really changing his plan, instead he is still in the planning process. But then taking verse 8 literally means we have to say God did have a plan. So we get an inconsistency by being consistently literal here! So I think we have to read “planned” as “devised” in verse 8, and we can note that “fashioning calamity” is parallel with “devising a plan” in verse 11, and that would explain the meaning, God is “taking off his belt,” getting ready to punish them, that is part of his plan, but the actual planned outcome is not mentioned.
But God does seem to show his actual plan, in the very next chapter:
JER 19:1-2 This is what the Lord says: “Go and buy a clay jar from a potter. Take along some of the elders of the people and of the priests and go out to the Valley of Ben Hinnom, near the entrance of the Potsherd Gate.”
These references to a finished pot, and shattering it at the Potsherd Gate, seem to intentionally reference the analogy of the potter in chapter 18. This would show that God’s plan is actually settled, even while Jeremiah is warning them. And the whole analogy of the potter forming and then reforming the clay suggests that the potter is in control all the way through, clay doesn’t change itself, that could not be what caused the potter to stop shaping it one way, and start shaping it differently.
Rom. 9:21 Does not the potter have the right to make out of the same lump of clay some pottery for noble purposes and some for common use?
And here again, we see a purpose already determined, not a choice that might vary, one way or another, “out of the same lump of clay,” the purpose is there from the start.