You're not going to like my story. My daughter has always preferred my left boob. She is 17 months old now. While she will occasionally nurse from the right, sometimes when I offer it to her, she will tell me "no." I asked about this at a breastfeeding support class when she was about 7 months old, and there was a woman there with a much older baby who said that she was only producing out of one side.
I don't know if any lopsidedness is irreversible, but I've always been lopsided even before babies anyway. Of course, it's more noticeable now that I'm nursing, but hopefully it's not too obvious afterward. But right now I call my boobs "California," because my left is a C and my right is an A. I have always heard, though, that it's not the nursing that stretches out your boobs but the expansion during pregnancy. It could be something that pro-nursing advocates have made up to convince women to continue breastfeeding, but right now I choose to believe it. :)
Good luck.