"full-upgrade" a debian to "bullseye"

debianupgrade.sh

#!/bin/bash

# many thanks to waja, parts taken from this gist: https://gist.github.com/waja/92947229661b2d07545adb29b992957e

dpkg-reconfigure locales

# are there 3rd party packages installed? (https://www.debian.org/releases/bullseye/amd64/release-notes/ch-upgrading.de.html#system-status)
echo "check for 3rd party packages..."
apt search '~i(!~ODebian)'

# check for ftp protocol in sources lists (https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.en.html#deprecation-of-ftp-apt-mirrors)
rgrep --color "deb ftp" /etc/apt/sources.list*

# Transition and remove entries from older releases
sed -i /lenny/d /etc/apt/sources.list*
sed -i /sarge/d /etc/apt/sources.list*
sed -i /squeeze/d /etc/apt/sources.list*
sed -i /wheezy/d /etc/apt/sources.list*
sed -i /jessie/d /etc/apt/sources.list*
sed -i /volatile/d /etc/apt/sources.list*
sed -i /proposed-updates/d /etc/apt/sources.list*
# change distro (please move 3rd party sources to /etc/apt/sources.list.d/), maybe look into http://ftp.cyconet.org/debian/sources.list.d/
sed -i s/buster/bullseye/g /etc/apt/sources.list*
sed -i "s/ stable/ bullseye/g" /etc/apt/sources.list*
sed -i s/buster/bullseye/g /etc/apt/preferences*
sed -i s/buster/bullseye/g /etc/apt/sources.list.d/*buster*
# Fix secuity suite (https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information#security-archive)
sed -i "s/bullseye\/updates/bullseye-security/" /etc/apt/sources.list*
rename.ul buster bullseye /etc/apt/sources.list.d/*buster*
rgrep --color buster /etc/apt/sources.list*

echo "updates sources list, please check! :)"
read -p "Press enter to update apt sources"

apt clean
apt update

read -p "Press enter to start full upgrade"
apt full-upgrade

echo "autoremoving..."
apt autoremove

read -p "press enter to reboot"
reboot