I don't know if there is a light side complexion effect ingame, but it is possible to change alignment. The sith warrior get two opportunities to reverse his alignment
which completely reverses it, light to dark and vice-versa.
You can also change it the hard way, by choosing the alignment you want, but if you leave this too late it won't work, ie if the choices you can make are less than your current alignment, but you won't know that unless you try. I would say if you have entered chapter 2 then you are probably better restarting.
Also is it worth it, the Jedi Consular story doesn't change, but the Sith Warrior story does, I would research how much your story will change, via the forums here and google. And if it's just the complexion, then you can turn it off.