Live Upgrage feature in Solaris OS, refer to live_upgrade(5), no doubt, is extremely convinient way of doing not only an operating system upgrade, but also a single product installation or the other various experiments - all of these in a very very safe manner.

The basic idea is to maintain several operating system images - "boot environments" on a single machine at the same time. Besides, boot environments may share partitions. So, the current environment can be smartly cloned to the other spare partitions, then modified as it is required - starting with sevral files replacement, and ending with the whole OS level upgrade. After that, a newly created alternative environment can be used as a working one.

If something is screwed up, you can fall back to your previous stable boot environment any time.

Continuing with Live Upgrade, you may clone boot environment 1 to 2, upgrade 2, reboot into 2, thus 2 becomes your current environment, and so you may clone 2 back to 1, do upgrade, switch to 1 and so forth - i.e. you'll get a non-stop safe upgrade process. [Read More]