Answer: It didn’t. Lincoln’s goal was always to preserve/restore the Union.
What changed was Lincoln’s stance toward slavery. At the start of the war, Lincoln expected to end the war and leave slavery as it stood: intact in the states where it existed, but locked out of the territories. Midway thru the war, ending slavery became a war aim.
- The Confederacy was tough to beat, and their slaves were a help to them. Freeing slaves was a means to hurt the Confederate war effort.
- Thousands and thousands of slaves flocked to the Union army as it traveled thru the South, and the govt had to develop a consistent policy toward them.
- There was a risk that Britain could “recognize” the Confederacy. The Emancipation Proclamation made it very difficult for the British government to come out on the side of the Confederacy. Britain had anti-slavery laws, and the British people were very staunchly anti-slavery. So “realpolitik” played a part.
- A hundred thousand black troops were fighting as Union soldiers. Lincoln felt strongly that the Federal govt couldn’t “break faith” with them by sending them back into slavery at the end of the war.
- Slavery caused the rupture that started the war. If slavery survived the war, the same rupture could happen again. By the time the war was in its third year, and had demonstrated how awful it was and how many lives it could claim, it began to be clear that ending slavery was the only way the war could be “worth it”, in the sense of having accomplished something and never needing to be fought again. This was the impetus for the big push for the 13th Amendment, in the last year of the war.