No one is in favor of changing everything in our original Constitution. However, the Founding Fathers themselves said that they were abandoning the Articles of Confederation and adopting a new Constitution in order to form "a more perfect Union". The Founding Fathers were the first ones to recognize that neither the Articles of Constitution nor the Constitution created "a perfect Union", but that the new document simply formed "a more perfect" one.
They knew that, as time went on, some things would have to change. Indeed, Slavery was deliberately left by the Founding Fathers as a matter to be settled in the future, since they couldn't resolve it among themselves.
In more recent years, other issues have arisen -- including One Man One Vote, Desegregation of the South, the right to organize labor unions, abortion, school prayer, affirmative action, burning the American flag, legal rights for Gitmo detainees, and so many more.
Thomas is always on the Conservative side of these issues. It is not because he wants to preserve the "original Constitution". I have tried to show you in my previous response that this is a phony argument since there are quite a few things in the "original Constitution" that no one would want to preserve -- having state legislatures, instead of the people, elect US Senators -- to name but one more. It is, rather, because Thomas has a Right-Wing Conservative viewpoint on so many of today's issues. He votes his political philosophy, not his supposed view that he wants to preserve the "original Constitution".
As you so aptly pointed out, Matt, "why would a black man want to count himself as only 3/5 of a person?"