I'm familiar with this theory and on some issues maybe it's relevant but what I'm referring to here is that Democrats today still seem to be in agreement with Democrats of old on the topic of whether the founders meant to abolish slavery.

Republicans say they absolutely did, just as they always have.

Democrats say they absolutely didn't, just as they always have.

I would argue that realignment occurred geographically, not on the basis of morality. Under slavery, the southern states were rich and mostly Democrats, now the northern states are more largely rich and leaning Democrat. That's to be expected, I think, where the wealthy wouldn't enjoy the new found poverty of the southern states as they rebuilt after the war, and would take their ideals with them. But that's just my guess, I don't have any research to substantiate that. Maybe it's an interesting topic for research.

Similarly, the stance on issues of whether people are naturally born inferior and deserving of special treatment, good or bad, remains a largely Democrat ideology, just as it always has.

Republicans on the other hand argue that all man is created equal not equitable, and they used that rhetoric to free slaves, stop Jim Crow era horrors, etc. And they continue to use it to argue against race based government aid.

So on these specific topics, I don't see any realignment as objectively observed.

All of this was and is documented in many SCOTUS cases, old and new.