If you have a place where you emit sum types, and you increase the set of possible cases you can emit then you do have to cross-the-entire-codebase to update all consumers of that sum type to handle the new case.

If you have a place where you consume product types, and you increase the set of fields that you inspect on that sum type, then you do have to cross-the-entire-codebase to update all producers of that product type to emit the new field.

I don't see how it could be any different. How would you handle this in Clojure?