IMHO its perfectly fine to change a rating at a later time if new information are available. Lets say you are happy with a trade and leave a positive feedback. Now you find out that the account was bought and you think that buying accounts is about the worst thing one can do here. Why should you not change your rating? Should you add a negative instead? You cant take a rating without context. Apparently for smooth its important to give out only a few ratings that have an impact.