Comment # 19 on bug 1223167 from Stefan Hundhammer

/var/lib/rpm/Packages is the RPM database. If that cannot be read, the upgrade
will fail for sure. So this is indeed a case for a zypper upgrade.

My interpretation of the logs so far was that it could not find a base product
on the installation media, but maybe it's indeed already at the point where it
would read the RPM DB of the previously installed product, and it's in reality
that which fails.

If you have a separate /var on ZFS, this cannot be mounted, so it falls back to
/var on the root filesystem, where in your scenario of course there is no RPM
DB. That would explain the behavior.