Use hashtable for everything. So Person who has a name address age and occupation is represented as a hashtable with those fields. When you want to add a new field like emailaddress it's just another key in the hashtable.

Ideally you want to code the person accessors so that they still specific externally

person-name person-age

the rest of the program uses these so it becomes easy to move person to a different structure if that's what you want .

You want to write some function to read a hash-table and write on to disc. In lisp this is trivial, but most languages have some sort of serializer. Then a function to load all the hash tables up when you start up.

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.