upgrading from ubuntu 12.04 to debian testing or sid

A good idea is to install a light desktop manager like lxdm or xfce so at least you have a DM in case
gnome will not work out

Install debian keyring http://packages.debian.org/wheezy/all/debian-archive-keyring/download


modify /etc/apt/sources.list
delete all the lines and
add these ones 
deb http://ftp.us.debian.org/debian sid main contrib
deb-src http://ftp.us.debian.org/debian sid main contrib

apt-get update
apt-get -f -o Dpkg::Options::="--force-overwrite" dist-upgrade


there will be a lot of issues and it might bring your  machine unbootable
So be prepared with a testing debian stick or cd

Here are some of the issues that could happen while upgrading the machine there are many dpkg-new files in the /etc/init.d/ folder so here are some tips bellow

Init scripts and networking system
Get rid of upstart first. I had problems with sysv-rc and needed to force reconfiguration. Some init scripts were installed with "dpkg-new" in the name, I had to rename them. As with X, I tried few solutions, this is distilled but stil redundant version:

dpkg-reconfigure sysv-rc
cd /etc/init.d
mv acpid.dpkg-new acpid
mv anacron.dpkg-new anacron
mv atd.dpkg-new atd
mv cron.dpkg-new cron
mv dbus.dpkg-new dbus
mv module-init-tools.dpkg-new module-init-tools
mv procps.dpkg-new procps
mv rsyslog.dpkg-new rsyslog
mv udev-mtab udev
dpkg --purge --force-all base-files ifupdown initscripts netbase sysv-rc sysvinit sysvinit-utils
apt-get --reinstall install base-files ifupdown initscripts netbase sysv-rc sysvinit sysvinit-utils

http://virtually-a-machine.blogspot.ro/2010/09/live-migrating-ubuntu-to-debian-howto.html