So the Lord changed His mind about the harm which He said He would do to His people (Exodus 32:14).
I have already commented on the issue of God "changing His mind" in an earlier post on this reading; however, here I want to address the question of the nature of this change. Notice, what God originally intended:
The Lord said to Moses, “I have seen this people, and behold, they are an obstinate people. Now then let Me alone, that My anger may burn against them and that I may destroy them; and I will make of you a great nation" (Exodus 32:9-10).
The original plan was to destroy literally everyone, which I presume includes men, women, and children. I say this because God tells Moses that He would build a new great nation "of you". In other words, just like He rebuilt the world through Noah, He would rebuild Israel through Moses. Now, this would put a swift end to the idea of the Twelve tribes of Israel, for Moses only represented one tribe, namely, the tribe of Levi. Therefore, I don't think this was God's true intention. Just as God tested Abraham by commanding him to sacrifice Isaac, it may be that God was testing Moses here.
Now, in spite of Moses' intercession, God would still judge the people for their sin. This is why we read,
The Lord said to Moses, “Whoever has sinned against Me, I will blot him out of My book. But go now, lead the people where I told you. Behold, My angel shall go before you; nevertheless in the day when I punish, I will punish them for their sin.” Then the Lord smote the people, because of what they did with the calf which Aaron had made (Exodus 32:33-35).
Now, it appears that God only judged some of the people. Even Moses killed 3000 of them with the sword in his anger. It appears that when Moses actually saw the people sinning, He had a better sense of God's anger over their sin. Therefore, while God agreed with Moses in relenting while on the mountain in view of His promises, Moses agreed with God in judging when he came off the mountain.
Nevertheless, Moses continued to intercede for the people that remained (Exodus 32:30-32), and God affirmed His justice to punish all those deserving punishment "in the day when I punish". Thus, while God did change His mind about immediately destroying all the people, He does judge some of them immediately and the rest over time.