We have a very large user base that was imported into Discourse. Most will not log on within 3 days of their Discourse IDs being created (it was more than 3 days before Discourse went into production). Their user names were created from this import. Our support staff does not have the time to field legitimate change requests that come in after the 3 day window so the decision was made to allow users to change their own IDs after more than 3 days.