NixOS: purely functional system configuration management

I failed to understand how this distro helps with managing configuration files. "upgrading a configuration is as safe as installing from scratch" makes me think that it is completely my responsibility to copy all my configuration files to the new version of a program being upgraded. I'm not sure it is correct understanding... Does this distro have any helpers to merge my changes in the config file with the upstream ones? Is it managed in "functional way"?