I started with the official docs (https://www.debian.org/releases/stable/i386/release-notes/ch-upgrading.en.html) and followed them through the first reboot. At this point I found myself with a black screen and I couldn’t get to VT1 or VT2. Booting to recovery mode worked, so I could read the logs.

nVidia drivers
In the end I had to entirely remove the nvidia-related packages that came from Debian, then download and run the installer from the nVidia website. I spent a bunch of time trying to avoid this since it shouldn’t be necessary, but in the end it was the approach that finally worked. I used this page: http://www.allaboutlinux.eu/remove-nouveau-and-install-nvidia-driver-in-debian-8/

More black screens
They weren’t over yet. In kern.log I found “NVRM: failed to copy vbios to system memory”. I found reports that kernels in the 3.10 to 3.18 range have issue with nVidia graphics. The Jessie kernel is 3.16 and I wanted to stay with a stock Debian kernel. Fortunately there is a workaround, the rcutree.rcu_idle_gp_delay=1 kernel parameter. This goes in /etc/default/grub. I added it to GRUB_CMDLINE_LINUX_DEFAULT:

GRUB_CMDLINE_LINUX_DEFAULT="quiet rcutree.rcu_idle_gp_delay=1"

After then running ‘update-grub’ my laptop booted into graphics mode.

“Oh no! Something has gone wrong.”
Not that the story ends there; the message “Oh no! Something has gone wrong” appeared after I rebooted. But I was happy, because at least I was in graphics mode and I had a new error to chase. My research suggested this had something to do with the Gnome desktop. Earlier in the upgrade process one of the steps uninstalled the Trinity desktop environment that I use, so I reinstalled that, choosing its components in the configuration questions I asked. And then suddenly, *finally*, everything was back! My desktop settings and my custom menu were there, WiFi worked, printing worked, and I could begin to try out the upgraded system.

This was not the worst operating system upgrade I’ve done, but it was probably the worst Linux upgrade I’ve done. I start operating system upgrades by cloning to a new drive so I can swap back whenever I need to. That was a lifesaver here.