zfs snapshot rpool/ROOT/opensolairs@for-cloning
zfs clone rpool/ROOT/opensolaris@for-cloning rpool/ROOT/opensolaris-1
zfs set mountpoint=legacy rpool/ROOT/opensolaris-1
mount -F zfs rpool/ROOT/opensolaris-1 /mnt
Here I have replaced rpool/ROOT/opensolaris with rpool/ROOT/opensolaris-1. I yet edited /rpool/boot/grub/menu.lst by copying an OpenSolaris entry and changing the bootfs rpool/ROOT/opensolaris line in the copy to bootfs rpool/ROOT/opensolaris-1. In the end I had two different entries in grub boot menu.
I then did:
rm -rf /platform/
and rebooted. In the above one line I have effectively wiped out kernel and boot_archive. It came as no surprise, that grub complained that it couldn't boot my old system. Oh well, I choose the second entry, booted from the clone, promoted it with zfs promote, destroyed the old system, which became the clone now, and type to you quite happily from working OpenSolaris 2008.05.
Have a Nice day.