God does not and has not changed. The God of the first testament is the same God of the second testament. He is just as just and just as merciful. Sometimes the God we see written about in the first testament is very difficult to understand, especially when there are verses that seem that God is being too harsh on people who don't have a chance. There is a very clear picture of the just side of God. The merciful side of God is also very evident. When the second testament comes into play, we are surrounded with love and mercy. (These are very very quick sketches). In some sense God appears to be different, but I know this to be not true.
When a human is born he enters a unending cycle of learning. From the moment he starts crying he is flooded with information and he learns. This never stops, and the older he gets the more of life he sees which helps him to understand it better. He will never know all there is to know, but he does continue to learn. Maybe all of time is like one person's life. When 'time' was born there was Adam and then Abraham and Moses and God started with who he was and that 'Sin is Bad.' There was nothing written, only the experiences of a few men. As 'time' grew older, he learned more about God as God was continually teaching about himself and revealing himself. God revealed himself to the full extent in Jesus, and 'time' is still learning and understanding him. 'Time' will never have God figured out, because if he were to figure out and understand God, then God would cease being God