Please also refer to http://www.debian.org/releases/bullseye/releasenotes and use your brain! If you can’t figure out what one of the commands below does, this is not for you. Expert mode only :)

# Crossgrading ?!?
[ "$(dpkg --print-architecture)" == "i386" ] && echo "How about crossgrading to amd64 as described in https://stbuehler.de/blog/article/2017/06/28/debian_buster__upgrade_32-bit_to_64-bit.html?"

# upgrade to UTF-8 locales (http://www.debian.org/releases/bullseye/amd64/release-notes/ap-old-stuff.en.html#switch-utf8)
dpkg-reconfigure locales

# remove unused config file
rm -rf /etc/network/options /etc/environment

# are there 3rd party packages installed? (https://www.debian.org/releases/bullseye/amd64/release-notes/ch-upgrading.de.html#system-status)
aptitude 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*
rename.ul buster bullseye /etc/apt/sources.list.d/*buster*
rgrep --color buster /etc/apt/sources.list*
apt update

# check package status
dpkg --audit
aptitude search "~ahold" | grep "^.h"
dpkg --get-selections | grep hold
dpkg --get-selections "*" > ~/curr-pkgs.txt
 
# unmark packages auto
aptitude unmarkauto vim net-tools && \
aptitude unmarkauto libapache2-mpm-itk && \
aptitude unmarkauto $(dpkg-query -W 'linux-image-4.19.0*' | cut -f1)
 
# have a look into required and free disk space
apt-get -o APT::Get::Trivial-Only=true dist-upgrade || df -h

# check for a linux-image meta package
dpkg -l "linux-image*" | grep ^ii | grep -i meta || echo "Please have a look into https://www.debian.org/releases/bullseye/amd64/release-notes/ch-upgrading.en.html#kernel-metapackage!"
# record session
script -t 2>~/upgrade-bullseye.time -a ~/upgrade-bullseye.script

# install our preseed so libc doesn't whine
cat > /tmp/buster.preseed <<EOF
libc6 glibc/upgrade boolean true
libc6 glibc/restart-services string
libc6 libraries/restart-without-asking boolean true
EOF
/usr/bin/debconf-set-selections /tmp/buster.preseed

# update aptitude and apt first
[ "$(which aptitude)" = "/usr/bin/aptitude" ] && apt install aptitude && \
[ "$(which apt)" = "/usr/bin/apt" ] && apt install apt

# transition sshd port changes and adjusted authkeyfile
if [ ! -d /etc/ssh/sshd_config.d ]; then mkdir -p /etc/ssh/sshd_config.d; fi
if [ ! $(grep ^Port /etc/ssh/sshd_config | tee /etc/ssh/sshd_config.d/port.conf | wc -l) -gt 0 ]; then rm /etc/ssh/sshd_config.d/port.conf; fi
if [ ! $(grep ^AuthorizedKeysFile /etc/ssh/sshd_config | tee /etc/ssh/sshd_config.d/authorizedkeysfile.conf | wc -l) -gt 0 ]; then rm /etc/ssh/sshd_config.d/authorizedkeysfile.conf ; fi
# transition ssh changes
if [ ! -d /etc/ssh/ssh_config.d  ]; then mkdir -p /etc/ssh/ssh_config.d; fi
if [ ! $(grep "^ *Port" /etc/ssh/ssh_config | tee /etc/ssh/ssh_config.d/port.conf | wc -l) -gt 0 ]; then rm /etc/ssh/ssh_config.d/port.conf; fi

# minimal system upgrade
apt upgrade

# chrony update, modify the new config to our needs and place it where it is expected.
if [ ! -d /etc/chrony/conf.d/ ]; then mkdir -p /etc/chrony/conf.d/; fi; echo "pool 0.de.pool.ntp.org iburst" > /etc/chrony/conf.d/pool.conf

# full-upgrade
apt full-upgrade

# (re)configure snmpd
COMMUNITY="mycommunity"; \
if [ -f /etc/snmp/snmpd.conf.dpkg-new ]; then CFG=/etc/snmp/snmpd.conf.dpkg-new; \
   else CFG=/etc/snmp/snmpd.conf; fi
sed -i "s/^agentaddress.*/agentaddress udp:161,udp6:[::1]:161/g" $CFG
sed -i "s/public default/$COMMUNITY default/g" $CFG

# Migrate (webserver) from php7.3 to php7.4
apt install $(dpkg -l |grep php7.3 | awk '/^i/ { print $2 }' |grep -v ^php7.3-opcache |sed s/php7.3/php/)
[ -L /etc/apache2/mods-enabled/mpm_prefork.load ] && a2dismod php7.3 && a2enmod php7.4 && systemctl restart apache2; ls -la /etc/php/7.3/*/conf.d/
# php-fpm
tail -10 /etc/php/7.3/fpm/pool.d/www.conf
vi /etc/php/7.4/fpm/pool.d/www.conf 
systemctl disable php7.3-fpm && systemctl stop php7.3-fpm && systemctl restart php7.4-fpm
# nginx
rename s/php73/php74/g /etc/nginx/conf.d/*php73*.conf
sed -i s/php7.3-fpm/php7.4-fpm/g /etc/nginx/conf.d/*.conf /etc/nginx/snippets/*.conf /etc/nginx/sites-available/*
systemctl restart nginx

# transition docker-ce to bullseye package
DOCKER_VER="$(apt-cache policy docker-ce | grep debian-bullseye | head -1 | awk '{print $1}')" && [ -n "${DOCKER_VER}" ] && apt install docker-ce=${DOCKER_VER} docker-ce-cli=${DOCKER_VER}

# transition icingaweb2 to bullseye package
ICINGAWEB2_VER="$(apt-cache policy icingaweb2 | grep "\.bullseye" | head -1 | awk '{print $1}')" && [ -n "${ICINGAWEB2_VER}" ] && apt install icingaweb2=${ICINGAWEB2_VER} icingaweb2-common=${ICINGAWEB2_VER} icingaweb2-module-monitoring=${ICINGAWEB2_VER} php-icinga=${ICINGAWEB2_VER} icingacli=${ICINGAWEB2_VER}

# Remove libgcc1 so cpp-8 can be updated
apt remove libgcc1 && apt full-upgrade

# remove old squeeze packages left around (keep eyes open!)
apt autoremove && \
apt purge $(aptitude search ?obsolete | grep -v -E 'linux-image|mailscanner|check-openmanage|check-linux-bonding|webalizer' | awk '/^i *A/ { print $3 }') && \
apt purge $(aptitude search ?obsolete | grep -v -E 'linux-image|mailscanner|check-openmanage|check-linux-bonding|webalizer' | awk '/^i/ { print $2 }') && \
apt purge $(dpkg -l | grep etch | grep -v xen | grep -v unbound | grep -v finch | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep lenny | grep -v xen | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb6|squeeze' | grep -v xen | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb7|wheezy' | grep -v xen | grep -v  -E 'linux-image|mailscanner|openswan|debian-security-support' | awk '/^rc/ { print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb8|jessie' | grep -v xen | grep -v  -E 'linux-image|debian-security-support' | awk '{ print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb9|stretch' | grep -v xen | grep -v  -E 'linux-image|debian-security-support|icinga2|phpmyadmin' | awk '{ print $2 }') && \
apt purge $(dpkg -l | grep -E 'deb10|buster' | grep -v xen | grep -v  -E 'linux-image|debian-security-support|icinga2|phpmyadmin' | awk '{ print $2 }') && \
apt -y install deborphan && apt purge $(deborphan | grep -v xen | grep -v -E 'libpam-cracklib|libapache2-mpm-itk')
apt purge $(dpkg -l | awk '/^rc/ { print $2 }')

# for the brave YoloOps crowd
reboot && sleep 180; echo u > /proc/sysrq-trigger ; sleep 2 ; echo s > /proc/sysrq-trigger ; sleep 2 ; echo b > /proc/sysrq-trigger

### not needed until now

# randomize crontab
if [ -f /etc/crontab.dpkg-new ]; then CFG=/etc/crontab.dpkg-new; else CFG=/etc/crontab; fi
sed -i 's#root    cd#root    perl -e "sleep int(rand(300))" \&\& cd#' $CFG
sed -i 's#root\ttest#root\tperl -e "sleep int(rand(3600))" \&\& test#' $CFG

# migrate unattended-upgrades config, modify the new config to our needs and place it where it is expected.
# Keep LOCAL config if asked when upgrading (and run this snippet afterwards, when dpkg is not blocked anymore and choose 'package maintainer version' then, cause this is the one we are adjusting here)
if [ -f /etc/apt/apt.conf.d/50unattended-upgrades.ucf-old ]; then CFG=/etc/apt/apt.conf.d/50unattended-upgrades.ucf-old; else CFG=/etc/apt/apt.conf.d/50unattended-upgrades; fi && \
cp /usr/share/unattended-upgrades/50unattended-upgrades /tmp/ && \
MAIL=$(grep ^Unattended-Upgrade::Mail $CFG | awk -F\" '{print $2}'); sed -i 's#//Unattended-Upgrade::Mail ".*";#Unattended-Upgrade::Mail "'${MAIL}'";#g' /tmp/50unattended-upgrades && \
TIME=$(grep ^Unattended-Upgrade::Automatic-Reboot-Time $CFG | awk -F\" '{print $2}'); if [ "${TIME}" != "" ]; then sed -i 's#//Unattended-Upgrade::Automatic-Reboot-Time "02:00"#Unattended-Upgrade::Automatic-Reboot-Time "'${TIME}'"#' /tmp/50unattended-upgrades; fi
sed -i 's#//      "origin=Debian,codename=${distro_codename}-updates"#        "origin=Debian,codename=${distro_codename}-updates"#' /tmp/50unattended-upgrades && \
sed -i 's#//Unattended-Upgrade::Remove-Unused-Dependencies "false"#Unattended-Upgrade::Remove-Unused-Dependencies "true"#' /tmp/50unattended-upgrades && \
sed -i 's#//Unattended-Upgrade::Automatic-Reboot "false"#Unattended-Upgrade::Automatic-Reboot "true"#' /tmp/50unattended-upgrades && \
sed -i '/codename=..distro_codename.-updates/ s#^//#  #' /tmp/50unattended-upgrades && \
/bin/bash /usr/bin/ucf --three-way --debconf-ok /tmp/50unattended-upgrades /etc/apt/apt.conf.d/50unattended-upgrades && \
[ "$CFG" == "/etc/apt/apt.conf.d/50unattended-upgrades.ucf-old" ] && mv $CFG /etc/apt/apt.conf.d/50unattended-upgrades.ucf-save

## phpmyadmin
if [ -f /etc/phpmyadmin/config.inc.php.dpkg-new ]; then CFG=/etc/phpmyadmin/config.inc.php.dpkg-new; \
   else CFG=/etc/phpmyadmin/config.inc.php; fi
sed -i "s/\['auth_type'\] = 'cookie'/\['auth_type'\] = 'http'/" $CFG
sed -i "s#//\$cfg\['Servers'\]\[\$i\]\['auth_type'\] = 'http';#\$cfg['Servers'][\$i]['auth_type'] = 'http';#" $CFG

# Update old postfix configurations
cp /etc/postfix/main.cf /tmp/main.cf && \
if [ $(postconf -n smtpd_relay_restrictions | wc -l) -eq 0 ]; then sed -i '/^myhostname.*/i smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination' /etc/postfix/main.cf; fi && \
if [ -z $(postconf -nh compatibility_level) ]; then sed -iE 's/^readme_directory = no/readme_directory = no\n\n# See http:\/\/www.postfix.org\/COMPATIBILITY_README.html -- default to 2 on\n# fresh installs.\ncompatibility_level = 2\n\n/' /etc/postfix/main.cf; fi && \
diff -Nur /tmp/postfix/main.cf /etc/postfix/main.cf && \
postfix reload

# Upgrade postgres
# See also https://www.debian.org/releases/buster/amd64/release-notes/ch-information.de.html#plperl
if [ "$(dpkg -l | grep "postgresql-9.4" | awk {'print $2'})" = "postgresql-9.4" ]; then \
 apt install postgresql-9.6 && \
 pg_dropcluster --stop 9.6 main && \
 /etc/init.d/postgresql stop && \
 pg_upgradecluster -v 9.6 9.4 main && \
 sed -i "s/^manual/auto/g" /etc/postgresql/9.6/main/start.conf && \
 sed -i "s/^port = .*/port = 5432/" /etc/postgresql/9.6/main/postgresql.conf && \
 sed -i "s/^shared_buffers = .*/shared_buffers = 128MB/" /etc/postgresql/9.6/main/postgresql.conf && \
 /etc/init.d/postgresql restart; \
fi
pg_dropcluster 9.4 main