Currently, it is possible to rename external users as long as the users are from the same external user directory.
However, sometimes it is necessary for an organization to swap to an entirely different user directory (e.g. OpenLDAP to Active Directory), containing a different set of usernames (e.g. "jsmith" to "john.smith"). This often occurs during mergers and acquisitions, for example. In this case, if the usernames are changed, Confluence has no way to link up the contents associated with old usernames to the new usernames.
This feature request is to request a tool to allow administrators to re-map content from one set of old usernames to new usernames, in bulk.