Part of the answer could lie into "what would someone teleported to another culture think ?" I don't think it totally solves the question, but it's a hint, or a part of the answer.

If you take someone from now, and he's teleported to dark ages, with absolute monarchy, serfdom, capital punishment with the most horrible ways of killing, torture, ... he will be horrified.

If you take someone from the dark ages and teleport him now, he'll probably be very lost at first, but I don't think he would be horrified by the fact we manage to take more-or-less reasonable decisions using democracy (at least as reasonable at what the kings used to do), that the society doesn't collapse into crime and chaos when we suppress death penalty, serfdom, torture, ...

Many people who, in the past, advocated the use of what we now consider barbaric (torture, death penalty, dictatorship, ...) did it saying "there is no alternative", "if we don't maintain order, it'll be chaos and everyone will murder each other", "if you don't have a king, no decision will be taken", ...

The same applies to points which are debated right now in western societies, like "painless" death penalty, or corporal punishment in education. People who are against them are horrified by them, people who are in favor are more arguing "we need them".

And it also applies things like prison, which are almost consensual right now. I find very few people around me who justify prison for the sake of it, but only because we need it to prevent/deter crime. So when we'll find a way to do without prison (or using it much less), by finding alternatives (using technology like electronic monitoring, societal evolution, better understanding of psychology and sociology, ...) people in the future will be outraged thinking how we locked people away for so long, while we, if teleported in that future, would be confused on how they manage to keep society from chaos without prison, but not outraged of it.

That gives a general direction of "ethical progress", which is (to a point) universal to all humans. But it's just a hint to a real theory of moral progress (I didn't read yet the following posts, nor took months formalizing it).