After that you be able to use steps below for upgrade your system to new BE.
Create new BE:
Mount newBE to temporary directory (for example we will use /a):
Update /a/etc/apt/sources.list by correct APT repos.
We will use dilos-unstable for example and put it on top:
deb http://apt.dilos.org/dilos dilos-unstable main contrib non-free
deb http://apt.dilos.org/dilos dilos-testing main contrib non-free
Update files list from APT:
apt-get -R /a dist-upgrade
Update boot archive:
bootadm update-archive -R /a
Reboot system to newBE: