Steps to be performed to upgrade from Lenny (amd64) to Squeeze. I don't explain why a certain step is done - read the manual found at step one. You should know what you're doing!

preperations

• read http://www.debian.org/releases/squeeze/amd64/release-notes/index.en.html
• backup your data
• backup as recommended:

tar czf ~/lenny-bkup.tgz /etc /var/lib/dpkg /var/lib/apt/extended_states \
/var/lib/aptitude/pkgstates
 
dpkg --get-selections "*" > ~/dpkg-selections.lenny

• prepare for recovery
• remove conflicting packages

apt-get purge splashy

• review actions pending in package manager: To perform this review, launch aptitude in “visual mode” and press g (“Go”). If it shows any actions, you should review them and either fix them or implement the suggested actions. If no actions are suggested you will be presented with a message saying “No packages are scheduled to be installed, removed, or upgraded”.
• checking packages status

# read http://www.debian.org/releases/stable/i386/release-notes/ch-upgrading.en.html
# to see why we do the following and what results these commands should give!
 
dpkg --audit
dpkg -l | pager
aptitude search "~ahold" 
dpkg --get-selections | grep hold

• update the package list

apt-get update

• make sure you have sufficient space for the upgrade
• minimal system upgrade:

apt-get upgrade

• upgrade the kernel and udev

apt-get install linux-image-2.6-flavor
# if you get an error message saying "W: Possible missing firmware ..." 
# you will need the nonfree firmware packages:
apt-get install firmware-linux-nonfree
 
update-grub
apt-get install udev

• upgrade the system:

apt-get dist-upgrade

• remove legacy-grub if grub 2 works well:

upgrade-from-grub-legacy

Done …

Resoving problems with insserv
if you encounter problems with insserv and have to resolv them manually you can initiate the change with dpkg-reconfigure sysv-rc