Just a quick tutorial on how to delete display name history on IP.Board 3.4.x.
Navigate to: ACP > SQL Management > SQL Tools
Scroll to the bottom of the page
Enter the following coding into the text box:
x = Member ID - You may find this in the URL of a users profile.
DELETE FROM dnames_change WHERE dname_member_id = x
Eg: If the username was 'Plum' and it was the 4th account to sign up to my website then the following would be it's profile ID: http://www.<WEBSITE>.com/user/4
To delete all display name history from every user type in:
Hit 'Run Query'
If done correctly it should say "Query executed successfully"
I do not take any credit for the coding provided in this tutorial.