gistfile1.php

function acf_fix_old_full_width( $field ) {
	$tdrfullwidth = get_post_custom_values('_tdrfullwidth');

	if( isset( $tdrfullwidth ) ) {
		if( $tdrfullwidth[0] === 'on') {
			update_post_meta( get_the_id(), 'full_width_header', 'parallax');
		}
		delete_post_meta( get_the_id(), '_tdrfullwidth');
	}

	return $field;
}
 
add_filter('acf/load_field/name=full_width_header', 'acf_fix_old_full_width');