When did the war change from a battle over preserving the Union to a war to end slavery

The correct answer is when he made the Emancipation Proclamation

At first, the war was fought to preserve the union. The northerners wanted to keep the seceding states in the union and they went to war to stop their independence. It was during the war that he made the emancipation proclamation which exclaimed that all slaves in the seceding countries were free and were welcome to join the northern army and this led to the three amendments that were introduced after the war.

The Civil War was always about slavery, but one could argue that Lincoln's idea of the war shifted slightly when he issued the Emancipation Proclamation, since this ended the possibility of slavery's preservation for good.