Crossgrading Ubuntu 18.04 to Debian 10

After upgrading all Ubuntu 18.04 packages to their latest releases, I adapted eudoxos‘ recipe:

Rebooted and selected the stock 18.04 kernel instead of HWE, which I was using
Downloaded debian-keyring and debian-archive-keyring .debs and installed them with dpkg -i debian*.deb
Created /etc/apt/preferences.d/10-no-ubuntu to pin down Ubuntu packages:
```
Package: *
Pin: release o=Ubuntu
Pin-Priority: -1000
```

Added Debian sources for buster to the end of /etc/apt/sources.list:

deb http://deb.debian.org/debian buster main contrib non-free
deb http://deb.debian.org/debian-security/ buster/updates main contrib non-free
deb http://deb.debian.org/debian buster-updates main contrib non-free

apt update && apt-get dist-upgrade
Answered “no” to all configuration changes questions (I’ll update them later)
Post-install, networking was non-functional. Apparently this was caused by Apparmor, so I disabled it (systemctl disable apparmor.service) and rebooted.
Deleted /etc/apt/preferences.d/10-no-ubuntu
Then it was time for manual package surgery. Lots and lots of it. Searching for remaining Ubuntu packages is easy enough, with
```
dpkg -l | grep ubuntu
```
and
```
aptitude search '?narrow(?installed, ?not(?origin(Debian)))'
```
Many of the matching packages can be removed/downgraded to their Debian versions without issue, but a bunch of gcc and python packages turned out to be tricky, and had to be downgraded all together. I did make notes, but following them blindly would most likely just do more harm than good, so I won’t post them here; you’ll need to hack’n’slash your own way anyway.