Upgrade Ubuntu option not availiable when installing from USB stick!

Upgrading from the install medium is only available if you have internet access during the install. The source you are referring to had internet access during the install, that is why he had that option. But gaining internet access during install won't help in your case, because you don't want to rely on your not reliable internet connection.

Official source for the above: https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1170531

Dimitri John Ledkov (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.