On DailyKos and elsewhere a significant amount of the conversation concerning modification or demolition of the Stone Mountain confederate memorial has revolved around the principle of being against destroying artwork, and against destroying artwork for political or ideological reasons.
I propose another way of looking at it and I will use the analogy of graffiti (and other forms of unauthorized muraling).
Many people seem to hold onto a platonic notion of art that completely denies the context of how the art enters and is maintained in the world. When someone paints onto a wall that they do not own or starts defacing a prominent mountainside, non-consensual imposition is inherent to the art's creation and ongoing existence.
White racist fascists unilaterally acted to deface a geological formation that had existed for millions of years in order to prominently display fascist propaganda that celebrates those who killed and destroyed to preserve human enslavement. That geological formation is now public property.
Now that it is public property then, same as a business owner who owns a building has the right to paint over racist messages and symbols that are tagged on the walls of their store, American citizens of African slave descent and all others with an interest in social justice have the right to demand that the Stone Mountain bas relief be removed by any means necessary from public property.
This bas relief was intentionally made as a unilateral imposition upon a prominent public place by white fascists and their enablers. Consequently, members of the public, in the interest of social justice and in opposition to racial oppression, are in their rights to strike back against this imposition on the public space with modification or destruction of the violating art.
There is a priority in preserving art, yes, but it is unethical for art to be imposed on people against their consent. When such a violation of consent occurs, it is the right of the afflicted party to seek redress.
Just a final thought, there are those who would bemoan that demolition is the only way to achieve removal of the sculpture. They would blame those seeking removal for the destruction of artwork. But the fact is the responsibility for that reality lies squarely with the racist fascists who willfully acted to make a bas relief sculpture instead of a statue arrangement inside of a museum, confederate graveyard, or other memorial area.