What filter are you trying?
If you use acf/update_value then $value will be the new value and the old value will be in the DB.
If you use acf/save_post with a priority of <10 then the old value is in the DB and the new value is in $_POST[‘acf’][$field_key]
ACF does not have the old value, nor care what it is, when saving. ACF will never have the old value when updating a field.
