Currently, it is not possible to change a username for an ArcGIS Online account.
When there is more room to add users into an organization, an administrator can invite a new user and change the owner of the content to the new user. If there are no more spaces for new users to add an additional user, the administrator must delete a current user, and invite the new user.
To not lose all the content, the administrator must change the owner to move the content to another username before deleting the current user. After deleting the current username, invite the new user with the desired username, and transfer the content again to the user it belongs to.