what do you call the natural change seen occuring in other cultural artifacts, such as the law? How about the legality or illegality of slavery?

There are several schools of jurisprudence which ask questions about how the law changes over time (and from place to place). They are remarkably similar to the schools of prescriptivism and descriptivism in linguistics. In analytic jurisprudence, the emphasis is on what the law is, not what it ought to be. It is neutral on issues of morality. As regards slavery, analytic jurisprudence would say something about which legal systems promote it, which tolerate it and which prohibit it (and how) without asking the question "Is slavery a good thing or a bad thing?" This school, it seems to me, is the jurisprudential equivalent of descriptivism. In normative jurisprudence, the emphasis is on what the law ought to be, not what it is. As regards slavery, normative jurisprudence would assert some values or first principles -- e.g. the dignity of every human being, the legal equality of every human being -- and reason from them that slavery is wrong. This school, it seems to me, is the jurisprudential equivalent of prescriptivism. A legal system which moves from permitting slavery to prohibiting it would be seen as improving, from a normative perspective, but only changing, from an analytic perspective. I have oversimplified both of these schools, but one ought not hog too much space in a thread which is not really about jurisprudence.