Before reading this upgrade guide, please note that live updates to your production servers are carried out at your own risk. Debian Edu/Skolelinux comes with ABSOLUTELY NO WARRANTY, to the extent permitted by applicable law.
Please read this chapter completely before attempting to upgrade.
General notes on upgrading
Upgrading Debian from one distribution to the next is generally rather easy. For Debian Edu this is unfortunately not yet true as we heavily modify configuration files in ways we shouldn't. (See Debian bug 311188 for more information.) Upgrading is still possible but may require some work.
In general, upgrading the servers is more difficult than the workstations and the main-server is the most difficult to upgrade. The diskless machines are easy, as their chroot environment can be deleted and recreated, if you haven't modified it. If you have, the chroot is basically a workstation chroot anyway, so rather easy to upgrade.
If you want to be sure that after the upgrade everything works as before, you should test the upgrade on a test system or systems configured the same way as your production machines. There you can test the upgrade without risk and see if everything works as it should.
Make sure to also read the information about the current Debian Stable release in its installation manual.
It may also be wise to wait a bit and keep running Oldstable for a few weeks longer, so that others can test the upgrade and document any problems they experience. The Oldstable release of Debian Edu will receive continued support for some time after the next Stable release, but when Debian ceases support for Oldstable, Debian Edu will necessarily do the same.
Upgrades from Debian Edu Lenny
Be prepared: make sure you have tested the upgrade from Lenny in a test environment or have backups ready to be able to go back.
The basic upgrade operation
Edit /etc/apt/sources.list and replace all occurrences of "lenny" with "squeeze".
run apt-get update
run apt-get upgrade
run apt-get dist-upgrade
LDAP service needs to be reconfigured
The LDAP setup has changed a lot from Lenny to Squeeze. The way users accounts, passwords, network settings, services and machines are treated is quite different now. So LDAP has to be rebuild from scratch. There's a script ldap-debian-edu-install (in /usr/bin) that could be used to achieve this. Read the comment at the beginning of that script carefully before doing anything.
Recreating an LTSP chroot
On the LTSP server(s) the LTSP chroot should be recreated. The new chroot will automatically support both thin-clients and diskless workstations.
Remove /opt/ltsp/i386 (or /opt/ltsp/amd64, depending on your setup). If you have enough diskspace, consider backing it up.
Recreate the chroot by running debian-edu-ltsp as root.
Of course you can also upgrade the chroot as usual.
Upgrades from older Debian Edu / Skolelinux installations (before Lenny)
To upgrade from any older release, you will need to upgrade to the Lenny-based Debian Edu release first, before you can follow the instructions provided above. Instructions are given in the Manual for Debian Edu Lenny about how to upgrade to Lenny from the previous release, Etch, and the Etch manual covers the one before that!