Yes it is possible to change your username.
Go to the main forum page http://www.tarotforum.net/ and look in the bottom right hand corner of the page. There you will see a button titled 'E-mail us'.
Click on that and it will take you to a contact form.
You can then contact Solandia (the webmistress of this wonderful site) and let her know what you want to change your user-name to.
Simple as that :)