Two Covenants to Preserve the World

The biblical text implies that God created a covenant with the world when it was created. This covenant is implied in the two reasons for saving Noah from the flood the necessity of preserving a remnant of Creation based on God's first covenant with it, and the reward appropriate for the righteous man of the generation - which are reflected in the two components of God's revelations to Noah. If a covenant with the world already exists, why is there a need for another covenant after the flood? What does the rainbow covenant change or add?