Upgrade Debian 8 to 9

# Get latest Debian 8 updates
mv /etc/apt/sources.list /root/sources.list.old-jessie-backup
mv /etc/apt/sources.list.d/php.list /root/php.list.backup
echo "deb http://deb.freexian.com/extended-lts jessie main contrib non-free" > /etc/apt/sources.list
wget http://deb.freexian.com/extended-lts/archive-key.gpg -O /etc/apt/trusted.gpg.d/freexian-archive-extended-lts.gpg
apt-key add /etc/apt/trusted.gpg.d/freexian-archive-extended-lts.gpg

apt-get update
apt-get -y upgrade
apt-get -y dist-upgrade

# Prepare for Debian 9
mv /etc/apt/sources.list /root/sources.list.jessie-backup
echo "deb http://archive.debian.org/debian/ stretch main contrib non-free" > /etc/apt/sources.list
echo "deb http://archive.debian.org/debian/ stretch-proposed-updates main contrib non-free" >> /etc/apt/sources.list
echo "deb http://archive.debian.org/debian-security stretch/updates main contrib non-free" >> /etc/apt/sources.list
echo "deb http://deb.freexian.com/extended-lts stretch-lts main contrib non-free" > /etc/apt/sources.list.d/extended-lts.list
wget https://deb.freexian.com/extended-lts/pool/main/f/freexian-archive-keyring/freexian-archive-keyring_2022.06.08_all.deb && dpkg -i freexian-archive-keyring_2022.06.08_all.deb
find /etc/apt/sources.list.d/ -type f -exec sed -i 's/jessie/stretch/g' {} \;

# Let's run upgrade to Debian 9:
apt-get update
apt-get -y upgrade

# Let's run dist-upgrade to Debian 9:
apt-get -y dist-upgrade