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We would like the ability to install patches and version upgrades in a stand aside mode. This would need to happen on the "live" system. Basically, this install would copy the existing "root" file systems (i.e /, /usr, /var) onto the "new" disk. Upgrades and patches would be then applied to the "new" filesystems. A reboot would then be required to boot off the "new" system file systems. If problems arise you can then boot the "old" system and revert back to the system prior to the changes. This would be similar to the "Live Upgrade" of Sun Microsystem's Solaris.
We do not plan to support this mode of upgrades in the near future. If you wanted to do this you could put code in your %pre of a kickstart file to backup the filesystems, upgrade the existing, and if you don't like what happened copy the backup over the upgraded system.