I have installed at home Solaris Express build 55b and it works nice (although it takes a bit of time to boot) and wanted to play with live upgrade. Live upgrade lets you install a different build without deleting your documents. What’s best is that you can choose in which build you want to boot.
For that you need another partition that will be the alternate root. My slices are the following:
c0d0s0, /, 15000MB
c0d0s1, /altroot, 15000MB
c0d0s2, swap, 512MB
c0d0s3, /export/home, 15000MB
I modified /etc/vfstab so it does not mount /altroot automaticaly at boot and then created the liveupgrade environment:
# lucreate -c "solenv1" -m /:/dev/dsk/c0d0s1:ufs -n "solenv2"
Then I used the build 56 DVD to upgrade (you can also do this without burning the dvd by mounting the .iso):
# luupgrade -u -n solenv2 -s /cdrom/SolarisDVD
You need to activate the environment
# luactivate solenv2
And then reboot
# init 6
Now you’re having build 55 and 56 
More reading: Solaris Live Upgrade 2.0 Guide and BigAdmin – Live Upgrade
