Note that previous software versions also includes previous configuration files. I don't believe there's anyone who manages things like database schemas via Nix, you're right there.
EDIT: Thinking about it, it should be theoretically possible to write a schema migration layer on top of Nix. In the system activation script, copy all schema migration scripts to some directory (/var/schema or similar), then run the schema migration tool. When you try to rollback, all the schema migration scripts will still be in that directory, so the schema migration tool can rollback as well.
EDIT: Thinking about it, it should be theoretically possible to write a schema migration layer on top of Nix. In the system activation script, copy all schema migration scripts to some directory (/var/schema or similar), then run the schema migration tool. When you try to rollback, all the schema migration scripts will still be in that directory, so the schema migration tool can rollback as well.