Reading the jira documentation to rename a user.
This is not working
The issue :
JRA-4588 dont seems to be fixed.
To change a username:
Locate the user in the User browser (see Viewing Users above) and click their Username. This displays the user's details, below which are several links.
Choose Actions > Edit Details.
Edit the Username.
Click the Update button.
This is wrong, changing username does not show up.