I guess then we have to consider the Nazis to be moral. After all, their society decided that it was good to be rid of Jews. In truth I think this isn't the issue. We all know that there has to be some way to determine what is right or wrong. We can't just go with whatever strikes our fancy at any moment.
Morality is a standard.
If a standard is changable, then it is not a standard. It would be an exercise in futility. Actually finding tires to fit your wheel could happen only by fortunate accident. If morality is changable, then it is only a fortunate accident when an action you or I considers abominable is viewed by the rest of the world as such, for that day, at least. Fortunately for us, G-d gave us a moral system called the Torah that has revolutionized the world.