How can I perform an offline upgrade using Ubiquity on the installation disc?

I'm trying to upgrade an Ubuntu 12.10 installation to Ubuntu 13.04 without an Internet connection. Now that the alternate and desktop CDs have been merged, I'm trying to use the 13.04 Desktop CD in order to upgrade my 12.10 system. In Ubiquity, there is an Upgrade Ubuntu 12.10 to Ubuntu 13.04 option. So I guess this is doable.

Steps to replicate:

1. In a virtual machine, install a clean, fresh Ubuntu 12.10 system. Apply all available updates;
2. Shutdown the VM;
3. Disconnect network cable from the VM. Make sure your VM has no Internet link -- remember I want to do an offline upgrade;
4. Insert the Ubuntu 13.04 beta 2 installation ISO in the VM. Reboot the VM;
5. Select your desired language, and select Install;
6. On Preparing to install Ubuntu screen, make sure again that you see no Internet connection available. This prerequisite should be marked by an X on this screen. If an Internet link has been found by Ubiquity, disable this connection before continuing;
7. Then, on Installation type screen, observe that the Upgrade Ubuntu 12.10 to Ubuntu 13.04 option is greyed!!! It cannot be selected.

What is strange is the upgrade option as again available if an Internet connection is found by the installer. Repeat the previous steps, but this time, make sure your VM has an Internet link. Back on the Installation type screen, the upgrade option can be selected.

Is this a bug in Ubiquity? Is it normal -- the expected behavior -- for Ubiquity to require an Internet connection in order to perform an upgrade from Ubuntu 12.10 to 13.04?

(Please note that I did not have any problem doing an online upgrade using Software Updater from my current 12.10 install. That is not what I want to achieve -- I already wrote the online upgrade guide. What I want to know is: how can I perform an offline upgrade using Ubiquity on the installation disc?)

I posted a bug report about this in Launchpad. I got the following answer from Dmitrijs Ledkovs (xnox):

It is correct, we do not support offline upgrades via ubiquity/desktop CDs. And even for servers, it is not recommended to use new server CD to do offline upgrade.

The support upgrade methods are with internet connectivity or a local mirror / package pool. One can for-example use tools like aptoncd to snapshot all installed packages, download them on a machine that has internet connectivity, bring them to the offline machine and upgrade it.

The reason behind this move, is enourmous amount of support cases of broken and partial upgrades since newer installation media will be missing manual additionally installed packages; packages that have now transitioned to new names; packages that have been removed from the default installation. Thus default install media for next release simply doesn't contain everything one will reasonably need to upgrade, hence the internet connectivity requirement.

So here the answer: even when using the Ubuntu installation disc, one cannot upgrade the a newer version of Ubuntu without Internet connection. Ubiquity needs to have an Internet connection in order to make the upgrade option selectable.

I understand the reasoning behind that. The installation disc cannot have all the repositories' packages. If a user has installed additionnal softwares after he installed Ubuntu -- and he certainly has --, then without an Internet connection, Ubiquity would fail to upgrade every packages, update dependancies, resolve missing dependancies, etc.

I'm a bit disappointed, though, as because if this limitation a user who wants/have to proceed with an offline upgrade will need to grab another media. He can't just use the same installation disc/USB key he usually lend his friends for them to install; he will need to grad extra packages.

I'm not familiar with APTonCD. I'll look deeper into this avenue later.