Looking around at other ubiquity bugs we can see that apt-clone with the restore-new-distro option has not worked for lots of people for quite some time. As an example bug see bug 1152399 which is a collection of duplicate bugs where the "install" of Ubuntu failed when calculating the upgrade from a previous release to a later release of Ubuntu. This leaves people in a hard to resolve situation and subsequently due to that issue and this crash with apt-clone we have decided to remove the option to upgrade a release of Ubuntu from ubiquity.
Looking around at other ubiquity bugs we can see that apt-clone with the restore-new-distro option has not worked for lots of people for quite some time. As an example bug see bug 1152399 which is a collection of duplicate bugs where the "install" of Ubuntu failed when calculating the upgrade from a previous release to a later release of Ubuntu. This leaves people in a hard to resolve situation and subsequently due to that issue and this crash with apt-clone we have decided to remove the option to upgrade a release of Ubuntu from ubiquity.