How to change role titles in Bbpress?

Have you tried this plugin: http://wordpress.org/extend/plugins/bbpress-string-swap/ ?

Or if you want to code, you could use this snippet in which you can change names. You should insert it into your functions.php

add_filter( 'bbp_get_dynamic_roles', 'my_bbp_custom_role_names');

function my_bbp_custom_role_names(){
 return array(

        // Keymaster
        bbp_get_keymaster_role() => array(
            'name'         => __( 'Keymaster', 'bbpress' ),
            'capabilities' => bbp_get_caps_for_role( bbp_get_keymaster_role() )
        ),

        // Moderator
        bbp_get_moderator_role() => array(
            'name'         => __( 'Moderator', 'bbpress' ),
            'capabilities' => bbp_get_caps_for_role( bbp_get_moderator_role() )
        ),

        // Participant
        bbp_get_participant_role() => array(
            'name'         => __( 'Participant', 'bbpress' ),
            'capabilities' => bbp_get_caps_for_role( bbp_get_participant_role() )
        ),

        // Spectator
        bbp_get_spectator_role() => array(
            'name'         => __( 'Spectator', 'bbpress' ),
            'capabilities' => bbp_get_caps_for_role( bbp_get_spectator_role() )
        ),

        // Blocked
        bbp_get_blocked_role() => array(
            'name'         => __( 'Blocked', 'bbpress' ),
            'capabilities' => bbp_get_caps_for_role( bbp_get_blocked_role() )
        )
    );
}