Changing clefs is easy. Just highlight the section to move and copy it. Then go to the new clef and paste. While the entire section is still highlighted then press control+shift+ up/down if your going from bass to treble it's 2 up, if going from treble to bass its 2 down.
Editor's Note: Also see the FAQ page (http://www.noteworthysoftware.com/composer/faq.htm) for Clef Change Procedure (http://www.noteworthysoftware.com/composer/faq2.htm#newclef).