Eh? God did not change his mind about anything, in fact in the Old Testament, Ever since the fall in the Garden of Eden, humankind has been less than it was created to be. We are under a curse in result of death and need of a Savior to save us, In the Old Testament God talked about his promise, about a Messiah. You have an Old Testament of people who disobey God from the begin he created Adam and Eve, to the end before Jesus arrives to earth. God showed patience
Then You have new Testament where The birth, life and death of the Lord Jesus changed everything, Jesus was crucified for our sins, the Savior to save us. The day when Jesus will return, as the Lord, In judgment, he will condemn all the nations of the world for their sin. God’s holy nature does not change. He is the same God in the Old Testament as He is in the New Testament, the only thing is different is we are allow to repent our sins, before we weren't allow to, because there was no blood of a sinless person to clean our sins.