This code snippet will override a new template on a page called ‘contact’ if the new template file was found.

add_filter( 'template_include', 'override_page_template', 99 );
/**
 * This will always override the contact page template using a custom template
 */
function override_page_template( $template ) {

	if ( is_page( 'contact' )  ) {
		$new_template = locate_template( array( 'template_contact.php' ) );
		if ( '' != $new_template ) {
			return $new_template ;
		}
	}

	return $template;
}