Because of how Nix works, upgrading should leave your system in an identical state as if you had done a fresh install. You can even downgrade if you want. I've

Because of how Nix works, upgrading should leave your system in an identical state as if you had done a fresh install. You can even downgrade if you want. I've switched to unstable and back to 16.03 (and now to 16.09) without a reinstall and everything still works perfectly.