The major change between these two constitutions was that the federal government became much stronger under the Constitution.
Under the Articles of Confederation, the national government was very weak. There was no president. The states had more power than the national government. Under the Constitution, the national government came to have much more power. There was a president and a national judicial system. The national government was given supremacy over the states.
One detail that changed (but did not increase the power of the national government) is that the Constitution had a system of separation of powers that the Articles did not have. It had a bicameral Congress as well as an independent executive and judiciary.