Terminal values can change with education. Saying that the coherent extrapolated volition of 19th-century slave owners would have been racist is equivalent to saying that either racism is justified by the facts, or the fundamental norms of rationality latent in 19th-century slave-owner cognition are radically unlike our contemporary fundamental norms of rationality. For instance, slave-owners don't don't on any deep level value consistency between their moral intuitions, or they assign zero weight to moral intuitions involving empathy.

If new experiences and rationality training couldn't ever persuade a slave-owner to become an egalitarian, then I'm extremely confused by the fact that society has successfully eradicated the memes that restructured those slave-owners' brains so quickly. Maybe I'm just more sanguine than most people about the possibility that new information can actually change people's minds (including their values). Science doesn't progress purely via the eradication of previous generations.