If I create a Live CD/USB and boot from that, is there any way that an existing system on the hard drive can be upgraded by use of the command line, and not the Ubiquity installer?
Sure. It's fairly similar to most remote-fixes (namely fixing Grub) where you
chroot into the system but mount some the runtime elements from the Live environment over the top. It's quite easy but it sounds and looks horrific if you're not used to it!
The following assumes that / is the live environment and that
sdb1 is the disk where the actual install is (check that first with
sudo fdisk -l!). Other than that, it's just a pile of commands to throw in:
sudo mount /dev/sdb1 /mnt for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt$i"; done sudo chroot /mnt
By this point you're root on your actual system and you can fix a whole load of things. Obviously this is the point where you need to be careful what you do but as long as you're not running around
rming a load of stuff, you'll probably be fine.
You probably could run the graphical updater but why muddy the waters with silly Xauth issues? The following will update your install's apt databases, download the command-line updater (if it's not installed already) and run it. You're already root so
sudo isn't necessary but I'm including it just in case you've dropped to another user (it doesn't hurt)
sudo apt-get update sudo apt-get install ubuntu-release-upgrader-core sudo do-release-upgrade
When you're done, just reboot onto your main disk.