If you are still not using live upgrade, you need to make learning it a priority. It will save you hours and make your manager happy because it costs nothing and gives you a simple, good and fast method for regressing your changes. You just need a few (about 10) GB of free disk space, be it in your root ZFS pool, on an un-used disk, or even any slice on any disk in the system.
The Live Upgrade concept is simple: make a copy of your "boot environment", run the upgrade or patching against this copy (called the alternative boot environment), and finally activate it.
Creation of the new boot environment is done by running a few simple commands which copies and updates the files in the new boot environment, an operation that can (and does) take a considerable amount of time, but runs in the back ground while the system is up and running, with all services online and active.
The Live Upgrade commands comes from three packages that you should install from the target OS's install media - for example if you want to upgrade from Solaris 9 to Solaris 10, you install SUNWlucfg, SUNWluu and SUNWlur from the Solaris 10 media (or run the liveupgrade20 install script in Tools/Installers directory)
Then once this is completed, another command (luactivate) is run to confirm that the new boot environment must be activated on the next reboot. On SPARC systems, this process modifies the boot-device in the OBP, while on i386 systems it updates Grub with a new "default".
Then all that is left is to do the actual reboot. During the reboot some special files and directories will be synchronized one last time - this is because between the time the system was copied over to the clone, and the time when the reboot runs, various things could possibly change: People still log in and change their passwords, recieve and send mail, spool jobs to the printers, etc. The administrator could even create new login accounts! To deal with this, Live Upgrade will synchronize a pre-determined list of files and directories during the first boot of the new boot environment.
The list of files copied is available here, and can be customized by editing the /etc/lu/synclist file.
The Live Upgrade system has got the intelligence built in to allow the new boot environment to find the files in the old boot environment during the boot up process, so this is completely automatic.
Recent Solaris Express installations prepare for the use of live upgrade by automatically setting up a slice and mounting is as "/second_root", but you need to unmount it and remove it from /etc/vfstab before live upgrade will allow you to use it. If you don't have a free slice, make one (backup /export, unmount it, and create two smaller slices in its place, one for live upgrade and one to restore /export to). This will be cheaper than performing upgrades the traditional way.