gistfile1.php

function my_acf_load_field( $field ) {
	$tdrfullwidth = get_post_custom_values('_tdrfullwidth');
	if( isset( $tdrfullwidth ) ) {
		if( $tdrfullwidth === 'on') {
			$field['default_value'] = 'parallax';
		}
		delete_post_meta( get_the_id(), '_tdrfullwidth');
	}
	return $field;
}
 
add_filter('acf/load_field/name=full_width_header', 'my_acf_load_field');