As far as we know, this doesn't exist in the PC version. I've checked the file that controls all the appearances, and it only has a column for darkside texture transition. There is no indication that there is any way to do this in the PC version.
I too have almost always played the lightside, and I don't remember it happening whenever I played it on the PC. I do remember that I was shocked the first time I played evil, and my character quickly started to look horrible with a cracked face. So, I definitely believe that you would remember how your character looked, and that you would notice if it was different. All I'm saying is that the neither the files nor the capability to do this exist on the vanilla PC version. I've never heard of anybody doing lightside transitions on KotOR for the PC.
Maybe you lucked out with the first XBOX version and got one that was different?