Switch to Debian testing repository using the fastest mirror

TL;DR here's a script to pull all the below together (fastest mirror tested using netselect-apt):

To fully migrate to Debian's testing/unstable repos (testing is preferred, unstable is present in case of dependency errors) first backup /etc/apt/sources.list:

cp -a /etc/apt/sources.list ~/sources.list.bak

Then check the state of the sources.list (as there may be references to stable in sources.list.d/ - we'll repeat this after migrating to testing).

grep 'debian.org.*stable' /etc/apt/sources.list /etc/apt/sources.list.d/*

Add the non-free repos

sudo sed 's/main$/main non-free contrib/g' -i /etc/apt/sources.list

Rename entries for stable and jessie to testing:

sed -E 's# (stable|jessie)# testing#ig' -i /etc/apt/sources.list

Set apt's default release

sudo sed 's#APT::Default-Release "stable";#APT::Default-Release "testing";#g' \
    -i /etc/apt/apt.conf.d/99defaultrelease

The install should now be on testing. Verify by grepping sources for stable (there shouldn't be any results):

grep 'debian.org.*stable' /etc/apt/sources.list /etc/apt/sources.list.d/*

Update the repositories:

sudo apt update

Install apt-listbugs and apt-listchanges:

apt install apt-listbugs apt-listchanges

Verify the installation channel by repeating the install, but specifying the -t testing flag - the packages should report that they are already the latest:

apt install apt-listbugs apt-listchanges -t testing

Verification of the latest version:

# apt install apt-listbugs apt-listchanges -t testing
Reading package lists... Done
Building dependency tree       
Reading state information... Done
apt-listbugs is already the newest version (<cut>).
apt-listchanges is already the newest version (<cut>).
...

Now, dist-upgrade to fully migrate to testing:

sudo apt full-upgrade

Be sure to always check the packages that will be removed. Although testing is generally stable, occasionally this can erroneously include xorg. Removing xorg will prevent X windows from booting (fixes available here and here).

You can safely remove files under /etc/apt/preferences and subdirectories:

sudo rm /etc/apt/preferences /etc/apt/preferences.d/*

When the installation is complete, reboot:

sudo shutdown -r now