I'm finally done updating my and my families computers to Debian 9 (Stretch). Finally as I also count the Raspberry Pis that are used e.g. as printer server at home and Raspbian needed a bit longer for the release.
All the preparation before the upgrade, where useless. At least from the point of view that I did not need any of the backups as the dist-upgrade worked flawlessly fine. Just a quick edit of the sources files replacing jessie with stretch
$> apt update $> apt dist-upgrade
and some time. That was all I had to do.
Nevertheless it is good to have now an updated backup base.
All in all the performance on our older machines got a nice boost, due the new kernel version used in Debian 8. Which is a really nice thing for the aging hardware. And another reason not to have to buy new hardware :-)
Thanks to everybody in the Debian and Raspbian community who made this smooth upgrade possible!