Hey,
Just to follow up on this. A friend of mine was able to modify the code slightly, and this is the version which worked:
function my_acf_save_post( $post_id ) {
// get new value
$user = get_field( 'author', $post_id );
if( $user ) {
wp_update_post( array( 'ID'=>$post_id, 'post_author'=>$user['ID']) );
}
}
add_action('acf/save_post', 'my_acf_save_post', 20);