How would versioning work?

[Profpatsch]

I think I understood the structure of these examples, but one thing that comes up often in practice: versioned configuration.

That is, the program in question changes the types of some fields, removes some fields, adds some fields …

How would I write a dhall config that can handle this? Where the user should (if possible) be able to import a dhall file that corresponds to the version of the program one wants to generate the config for, and dhall will typecheck for that version number.

Of course on the dhall side there should be as little duplication as possible.