Thanks John. I’ve managed to resolve it with this code snippet as I don’t have any custom filters or actions.
function action_function_name( $field ) {
global $post;
$id = $post->ID;
$value = get_field('user',$id);
$args = array(
'ID' => $id,
'post_author' => $value,
);
wp_update_post( $args );
return $field;
}
add_action( 'acf/render_field/name=user', 'action_function_name', 10, 1 );