For the most part it's painless as normal.
For the 'big move' it's best to have just the main repository of the version you are upgrading to. Remove any other repos including old ones of the same lineage.
You don't need to remove any packages from 'other' repositories, however, unless you pin them they may get upgraded. The configuration files are linked to packages not repositories so there's no problem there.
It's normally best to ensure that
dpkg are updated early.
Always CHECK before you let the
apt-get dist-upgrade do it's stuff. Occasionally it'll get confused and decide it's best move is to remove large numbers of packages (annoying). If you manually install (upgrade) a few important packages first it'll normally get better. If you find it wants to remove half the system when you upgrade a particular package the best thing to try is something like:
apt-get install packageyouwanttoinstall packageyoudontwantremoved
apt-get will then try something different.
One of the dangerous this this time is the replacement of
systemd. You may want to put a rejection of
systemd into your preferences file until the rest of the upgrade is done so this change can happen with all new initscripts.
Obviously, make sure you have backups before you try this and you know how to restore them!
Oh, and read the Release notes
To "reject systemd" you can add a section to your
/etc/apt/preferences file. Though generally, you don't want anything in there during the upgrade and you should remove this afterwards. If you do this you will have to install
systemd explicitly as it'll assume you have chosen no to do the switch.
Pin: release o=debian
On a related note, I've just upgraded aother server from
Rule one don't skip wheezy, it go so confused I ended up using my backups.
It's a more generic server so it had different packages; the
apache2 packages are of note. Samba has been upgraded to V4, this will probably break a custom configuration, everything seems to still be there, just a little different. The Debian standards for Apache's configuration files have changed; if you don't upgrade all the config files it'll break; you should have followed the rules when you did your customisations. Once I moved them to the "right" files it was fine. If you haven't already upgraded to
squid3 the old
squid package doesn't exist in
jessie you will continue to run happily on the
wheezy version but you should upgrade to
udev whinged what the kernel was too old. (I forgot to reboot after upgrading to