Like I said, you can’t alter the format ACF uses to save the date value because that would cause ACF to not be able to display the fields for editing.
What you need to do is to figure out a way, if it is possible at all, to filter the values when the theme gets/uses them to match the format that is needed there. This depends on how the theme or another plugin is getting the value.
For example, if the theme or plugin is using get_post_meta() to get the value you could possibly add a filter on the "get_{$meta_type}_metadata" hook called in the WP function get_metadata_raw() to alter the value being returned.
