If Lincoln, Grant, Sherman(mostly, supposedly), and half a dozen other famous Union folks "changed with the war" and became emancipationist, why couldn't Longstreet change? Or a little for Nathan Bedford Forrest?
I think the premise is wrong because of postwar writings. IMO the Union men's position on race sometimes didn't change as much as the accepted history purports. It's easy to write after the fact and say you changed 180 degrees. Some did, many didn't. Union officers sometimes took side with ex-confederates to keep the freedmen in virtual bondage.
So I call BS on a lot of intellectually arrogant "accepted authors". History was what happened. Not our interpretation of what happened. Many can't keep their current mindset out of historical events.