dpkg-reconfigure ? No.. it's not that at all. In fact pacman already have some of those ideas, it tries to be safe when it keeps old package in /var/cache/pacman/pkg, but Nix goes further and tries to make sense of all those changes in the system.
I just see some other ideas that could be inspiring futures versions of pacman, that all: http://www.st.ewi.tudelft.nl/~dolstra/pubs/eupfcdm-cbse2005-final.pdf
