Although it's not supported I used this guide to semi automatically upgrade my daily driver.
You'll want to create a restore point with Timeshift, and ideally a system image using CloneZilla, because stuff is likely to break. I also backup my home directory with Back In Time and have a sync app for certain files within that.
I first created two Virtual Machines for eOS7 and eOS8 and then compared the eOS7 system after it was upgraded to 8 with a vanilla eOS8 setup, before I was confident enough to go ahead.
Apt sources compatibility
The main culprit as usual were third-party apt sources. In particular, if you have this hardware, I recommend to remove the AMD GPU drivers completely and reinstall them after the upgrade, to avoid kernel compilation issues.
Applications
I also lost a few applications (HeyNote and Albert), but their preferences were still there so simply reinstalling them made them work again.
Heynote's AppImage would no longer start:
./Heynote.AppImage [35483:0919/135315.749882:FATAL:sandbox/linux/suid/client/setuid_sandbox_host.cc:169] The SUID sandbox helper binary was found, but is not configured correctly. Rather than run without sandboxing I'm aborting now. You need to make sure that /tmp/.mount_Heynot9r0j9T/chrome-sandbox is owned by root and has mode 4755I fixed this by installing the HeyNote snap version.
Albert could not be found (not sure why but the executable was no longer on the system) and I installed the OBS version from their website which setup a new apt sources file.
backintime-qt was gone, a simple apt install restored it.
Other issues
My custom keyboard shortcuts are still running but they are no longer visible. I have a bunch migrated over from /org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom1-custom7 but they are not showing in the Custom shortcut settings anytime, as these are now stored at /io/elementary/settings-daemon/applications/application-shortcuts. It's possible to dump them by restoring the dconf database to a test-user and then using dconf dump with some parameters.
I use an app called Input Remapper to map my option and command keys (long story) and this stopped working after the upgrade, despite the app saying the mapping is in effect. I'll try and recreate it and see if there's an update for it.
That's it, relatively smooth for an unsupported setup.