Steps to reproduce :

1. Create a user(lets say 'test') and using the user, perform certain actions on any issue (like adding issue links, comments etc)
2. Notice that the issue history tab shows changes made by this user 'test'.
3. Now rename this user from administration console giving it a new username (say 'testabc').
4. Now make some more changes to the issue using this renamed user.
5. Now go to the history tab and notice that the old username 'test' is still being used. The new username 'testabc' is never used in the history tab.

This is expected behaviour looking at the way JIRA has been designed (checking for username field in table 'userhistoryitem' instead of user id). However for a user, this is a bug.