You can get all parent post id by get_post_ancestors
function.
add_filter( 'document_title_parts', 'change_wp_title', 20, 1 );
function change_wp_title( $title ) {
global $post, $paged;
$grappig = $title;
// 404
if ( is_404() ) {
$title['title'] = 'file not available';
} elseif ( is_singular( 'schedule' ) ) {
// get all parent post's id
$parents = get_post_ancestors( $post->ID );
$parent_titles="Shedule: ";
foreach ( array_reverse( $parents ) as $key => $parentpost )
{
$postdata = get_post( $parentpost );
$parent_titles .= $postdata->post_title . ' : ';
}
$title['title'] = $parent_titles . $title['title'];
} // End if().
else {
}
return $title;
}
Reference: get_post_ancestors