Change WordPress displayed author name for all comments

Whenever you change the display name Users >> Your Profile >> Nickname, and set  the "Display name publicly as", it will only affect the newly posted comment by you. Actually, you can change the displayed author name for all comments.

You can change the displayed name of comment author for all comments using this sql statement:

UPDATE `wp_comments` SET `comment_author`='old-name'
 WHERE `comment_author`=‘new-name’