Old username still shows in issue history after renameing a user.

Steps to reproduce:

1. Create a user
2. Create an issue and assign it to the user created
3. Navigate to Administration > User Namangement and change the username for the user
4. Go back to the issue created, and reassign it to another user and then assign it to the same user created earlier

Expected Results:
The Issue History tab will show the new username of the user

Actual Results:
The issue History tab shows the old username before the rename of the user. Even though the full name is changed as expected, however the username is no changed.