I mean it did start to happen when many veterans where still alive. A lot of guys made their post-war careers on playing nice in an attempt to get the occupation of the south lifted or save their reputations. Memoirs reflect that shift with every general saying "well I wanted slavery to end sometime, but couldn't betray my state/ it was every other general who wanted to preserve it" etc.

Longstreet is kinda the ultimate embodiment of this war and rehabilitation