Logged in as Admin, I am able to change the username of posts of bookpages, etc. but I cannot figure out how to change the username of a comment. The comment was posted anonymously, but the user does have an account.

If I try to change the name, Drupal refuses and reports that the username belongs to an existing user.

Is there a way I can modify the username of the comment post?

Comments

you can update it directly in the database. It's the only way I can see that it can be done with a D6 install. You would need to update the uid and the username.

The uid value in the table comment in database should be 0 if the user posted anonymously. To update this value to his actual user id, you need to execute this SQL query (supposing you are using MySQL):

UPDATE `comment` SET `uid` = 'whatever-is-the-user-uid' WHERE `comment`.`cid` ='id-of-the-comment';