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.
Solution 1: Burning the ISO of Ubuntu 14.04 and booting the DVD
If you burn the ISO image and boot with the DVD created, you'll be able to perform the upgrade. You just have to select the option for upgrading your current installation of Ubuntu 12.04. Your data will be kept and the setup process will try to reinstall the packages installed (even external ones), if possible (of course), at the end of the install process.
It is even possible to upgrade from a 32-bit edition of Ubuntu 12.04 to a 64-bit edition of Ubuntu 14.04 that way. Of course, in that case, no previous package can be reinstalled...
Of course, it is not booting from the ISO itself (see Solution 2 below).
Solution 2: Using an external disk drive with Virtual CD/DVD feature
If you have an USB external drive with virtual CD/DVD feature (some enclosures like Zalman ZM-VE300 allow that), you'll be able to boot on the ISO image mounted as if it was a real CD/DVD.
If you have an external disk drive, but no Virtual CD/DVD feature built-in, you might be interested by YUMI, MultiSystem or else.
Solution 3: Creating an USB boot disk from the ISO with "Startup Disk Creator"
You can create an USB boot disk from the ISO with "Startup Disk Creator" which is included in Ubuntu by default. Just type "Startup Disk Creator" in the Dash.
Solution 4: Upgrading via Internet [ISO not used in this case]
If you want to upgrade Ubuntu 12.04 to Ubuntu 14.04 without waiting July 2014, you can also use this command line in Terminal:
update-manager -d
However, the ISO won't be used in this case. Everything will directly be downloaded from Ubuntu repositories.
Note
Don't forget to backup your data before performing any upgrade. It's always better to have a backup even if you don't have to use it than having no backup when you really need one ;-)!
Best Answer
Canonical doesn't provide alternate-cd since 12.10, so an upgrade through the installation disk won't be available.
The solution would be to create an offline mirror of the ubuntu package repositories [1]. Once you have one, you can follow this procedure :
update-manager
anddo-release-upgrade
reads the file/etc/update-manager/meta-release
to find the location of the meta-release file. This points to the internet locationchangelogs.ubuntu.com
normally. And if you just mirror the package repos, the meta-release file isn't included. So we need to fetch it first:Store it, for instance on the root of the internal mirror or some other convenient location, and put the url to it in the "URL" value in the
/etc/update-manager/meta-release
. If you're upgrading to a LTS release, fetch themeta-release-lts
file too and repeat the process.Edit the meta-release file you just downloaded and substitute the external mirror address with the url for the internal mirror so all package locations match up. For me this was replacing
archive.ubuntu.com
withfile:///
and the path of the internal mirror. Make sure the file is readable via http (or file permissions if using file access to repo).Run
update-manager
ordo-release-upgrade
and the upgrade should work as you were using an internet mirror.