I think that there were some distinct realities that caused many soldiers to change their anti- emancipation thinking. One would have to be Lincoln's Emancipation Proclamation. Issued at the start of 1863, one of the effects of Lincoln's proclamation was to mobilize public opinion in areas that the Union army controlled to see slavery as a moral wrong. It was at this point that the Civil War pivoted from a political struggle to a moral one, in which the Union had the edge over the South. This helped to cause much in way of change in soldiers' approaches to the issue of emancipation and why they fought.
Another distinct reason that enabled many soldiers, especially in the Union Army, to change their views was because as the war reached a point in 1863 where victory began to pivot, African- American soldiers were seen as allies and resources. The reality was that once Gettysburg began to turn the tide for the North, anti- emancipation thinking changed as Confederacy momentum began to wane. Union soldiers were able to recognize the efforts of African- Americans in the war effort, helping to change their thinking from anti- emancipation to a different thought process. Combined with Lincoln's Proclamation, anti- emancipation thinking began to emerge in soldiers who understood the change in their approach towards the war and its end.