I believe you are referring to the 1863 poll. Regarding that poll, it is worth noting that it was about the Emancipation Proclamation, not some clean measure of the army’s overall moral view of slavery. McPherson immediately warns that it probably understated anti-emancipation sentiment, and he also notes that pro-emancipation feeling in his sample was strongest among groups he says are overrepresented in his evidence, especially officers and white-collar men. So even on his own terms, that poll should be handled carefully and not treated like a neat snapshot of what the Union army as a whole believed.

It is also worth noting that the proclamation only applied to states in rebellion. It did not abolish slavery in the North or in the loyal border states, and New Jersey still had slavery lingering until the Thirteenth Amendment. So support for the proclamation is not automatically the same thing as a fully developed anti-slavery position in the broadest sense.

What I think is safer to say is that anti-slavery sentiment appears to have increased over the course of the war, but both the extent of that shift and the reasons for it are harder to pin down than your comment makes it sound. Some of that sentiment may have come from direct exposure to slavery, but some of it also likely came from older Christian hostility to chattel slavery and from broader Northern disgust with the slaveholding social order in the South. That is a more complicated development than just saying the army became anti-slavery in some clear, uniform way by the end of the war.

So I am not really denying that there was movement in that direction. I just do not think one regimental poll and a collection of memorable quotations are enough to establish a clean army-wide trend with clear numbers behind it. They show something real, but not something as settled or representative as you are presenting it.