If you’ve migrated your site from earlier versions of Plone to the latest ones in the 4.x series, or else have started using plone.app.discussion earlier than that, you might have come across a situation where comments on your site show user IDs as authors rather than a user’s full name. If you see this anywhere, chances are your comment’s migration didn’t go according to plan, and that the Creator information didn’t get recorded correctly on the comment.
I’ve just written this on the Collective developer docs: http://readthedocs.org/docs/collective-docs/en/latest/misc/upgrade.html#fixing-creator-details-on-existing-comments
The script there should help by stepping through all comments on your site and fixing up the creator details as required. As mentioned in the comments beneath the script, the mileage you get may vary depending on the state of your comments. I found half my site’s comments were fine and half weren’t. Some of mine had no author name, email, or user ID in the relevant fields, and the script there checks for these (and the actual existence of a user after looking up what’s in the Creator property), so that should be enough to sanity-check so it won’t stomp over correct comments.
That said, if for some reason, you happened to have a user with ID david and full name of Administrator, also had a user with the ID of Administrator, and had a comment which had an incorrect Creator but did have the three fields of author_username, author_name, and author_email, then it won’t affect those comments and you might just have to manually fix them up.