changing user name attribute results in new user being created

Steps to reproduce :

1. Login to Confluence
2. Navigate to Confluence admin >> General Configuration >> User Directories
3. Add an LDAP directory on there with the correct credentials
4. Allow it to synchronize
5. After that go back to the User Directories page again and under User Schema Settings, change the User Name Attribute to another attribute. For example change attribute cn to sn
6. Synchronize again
7. Go to the Users page in Confluence administration panel and you will see additional users being created.

Expected behavior :

The username should be renamed to the new attribute that have been updated

Current behavior :

The username is not renamed and new users is being added.