If I understand your question you're wondering how to deal with older flat files when your app expects a newer data structure. If you're using s-exprs make you

If I understand your question you're wondering how to deal with older flat files when your app expects a newer data structure. If you're using s-exprs make your structure a list where the first element is a version identifier. If you aren't using s-exprs then use some other extensible format like XML.