Re: When to be concerned?
If your baby does not seem distressed, I wouldn't really worry about it. If you are certain that you have an oversupply, then you can block feed to reduce supply if you are not already doing so. If you are already block feeding, keep at it and be patient because it can take awhile to work.
Illness can change stools too, so you can look out for a fever and other signs of infection if you have reason to be concerned about that.
But if stool color/consistency is the only problem, I don't see any reason for concern. I would start to worry if the oversupply starts causing you or your baby excessive discomfort.
K. Sophia - Mama to my little lactivore, the amazing Mr. X (11/10).