If that one user re-registers you won't need to chagne any settings for anyone else - maybe just the post counts for that one member, and that's only if they are concerned about it.
Why that one member is missing I can't answer and again, as Rick noted - there are NO delete statements issued during the upgrade.
If you want to delete the database, create a new one and do the import again you can try and see if they come over in a new import.
Regards,
Brett Harris
Infopop Corporation
