Install it on a USB key. If you want to try the upgrade but have a perfect backup of 12.04 LTS, you can image your drive (depending on resources) with the "dd" utility. WARNING: be very careful with this. You can easily destroy things with dd.

From the USB install;

  dd if=/dev/{ubuntuDrivePath} of={someOtherDrive}/{backupFile}.img

This creates a 1 to 1 representation of your drive. For example,

  dd if=/dev/sda of=/media/data/backups/workstation.img

You will of course need to use an additional drive to store your backup.

To restore, boot into USB again, and execute:

  dd if={yourBackup} of=/dev/{ubuntuDrivePath}

Well, what you can do is boot from a live usb stick, mount your drive, move /* to /old/ and install afresh. If everything works as desired, you can move back your /home/user directory or selectively get your home directory back. If you don't like it, move everything back. Then you only need to restore grub, which should be easily doable using a chroot from the live media.

Thanks! Alas, that means I have to deal with a fresh install. Which I may do, but it's not the straightforward things I was hoping for.

What I'm really looking for is the sort of upgrade/rollback process I'm used to creating for my production apps. One-button upgrade, one-button rollback.

Looks like Ubuntu isn't there yet. I suppose the easy thing is just to get another disk, clone my existing setup, and do the upgrade. If I don't like it, I'll just swap in the backup disk.

Yes, a live CD / live USB stick.

Why not use a virtual machine? Then you can be worry free about your current setup.

My major issue with Ubuntu upgrades are hardware related, and another set are issues that only show up after a few days of use. So I'd rather try it for a week and then roll back if it's a net negative.

Uhm, don't do an inplace upgrade anyway, they suck. If you have /home on another drive, there's no "backup" or "restore" stage.

No they don't suck, they rock. This isn't windows, and I shouldn't have to lost my setup to upgrade to the latest distro.

Yes, they do suck. I regularly see and help people that are dealing with broken systems because of stalled/incomplete/broken in place upgrades.

Regardless, it's easy to not lose your setup, run the latest OS AND not do in-place upgrades.

Just put /home on a separate partition and you're done. Use dpkg to make a list and restore a list of applications (same with repos). A few files to backup and a few minutes to wait for the apps to redownload. Completely clean system. Same apps. All the while /home was untouched.

I've never had a single issue with the in place upgrades, and no putting home on another partition is not sufficient. Lots of stuff in /etc has been customized and that would need restored as well. I've been through half a dozen or so and they've always worked fine for me.

I find, sometimes, that doing an

#apt-get -f missing

fixes many of these post-upgrade problems. YMMV!