The only way to accomplish this (that I know of) is to use the acf/save_post with a priority of < 10. This means that your filter will run before ACF saves any values to the DB. You would access the fields in $_POST[‘acf’]. You would use these field keys in your update_field() calls using the field key (not the field name). When you are done unset the input array
unset($_POST['acf']);
This last step will prevent ACF from saving anything because nothing exists.
