Actually God does change in the ways he deals with his people. For example circumcision is a thing of the past, don;t you agree?
The only thing Paul was teaching is circumcision was never kept as a means to salvation. Thats what the Jews were teaching and what he was correcting. Circumcision is very much not a thing of the past, it has huge health benefits that is still practiced today.
God's character does not change and His law He wrote is a reflection of His character. Holy, good, righteous, perfect, just etc.