>One of the coolest areas of package management is that you can go back in time and boot up into a previous state of the system, all thanks to the new packaging system. To do this, simply open the boot menu, choose the boot volume, and select Latest state or a nicely time stamped ‘version’. Very cool.

New versions of programs may modify the user config files in a way that old versions may not be able to read those config files anymore and even corrupt them. How is this handled, if at all?