I've seen people amend reviews in the App Stores without being prompted, it certainly helps and they tend to mention that they are updating a previous review.

I've seen people amend reviews in the App Stores without being prompted, it certainly helps and they tend to mention that they are updating a previous review. As for contacting people, Apple don't make this easy. The only way i've done it in the past is to match up user names, if you can't do that then you have no way to respond to the review in any way. Hopefully Apple will change that in the future or at least allow developers to push the user towards support via email or similar.