This question already has an answer here:
I typically upgrade downloading the relevant ISO (not necessarily the alternate one), going offline and then running:
sudo mount -o loop ubuntu-11.04-alternate-amd64.iso /media/cdrom gksu /media/cdrom/cdromupgrade
...and then connecting and doing an update once the system is installed. I do this for stability and so I can schedule the upgrade and update independently (i.e. juggling time and bandwidth).
However, the Trusty Tahr release notes say:
Offline upgrade options via alternate CDs are no longer offered for Ubuntu Desktop and Ubuntu Server. Please ensure you have network connectivity to one of the official mirrors or to a locally accessible mirror and follow the instructions above.
Does this mean that an offline upgrade is impossible, even with the non-alternate ISO? If it is impossible, can I still do the upgrade via a mounted ISO, or is a USB/DVD now required?