Raspbian Jessie to Stretch upgrade

Upgrading from Jessie to Stretch can break the entire system so ALLWAYS do a backup.

There are several sites/posts with "how to" information but the Raspberry site, the Linuxconfig site and the Superuser site, (where You can find more details about the upgrade), made the base for this post. 

Start with an upgrade of Your current system

$ sudo apt-get update
$ sudo apt-get upgrade
$ sudo apt-get dist-upgrade

Do the backup, using this script

After checking that the backup works start the upgrade changing every occurrence of "jessie" to "stretch" in the 2 files with these commands

$ sudo nano /etc/apt/sources.list
$ sudo nano /etc/apt/sources.list.d/raspi.list

and then the up-date/grade commands

$ sudo apt-get update
$ sudo apt-get upgrade

and answering  ‘yes’ to any prompts

Please check remark below !

do a sudo reboot

following the upgrade gave this information 

The following packages have been kept back:
  alsa-utils apt apt-listchanges apt-transport-https............

Which means that there are new versions of these packages which will not be installed due to broken or new dependencies. To solve this run

$ sudo apt-get --with-new-pkgs upgrade

After this update, check that there is no more packages kept back with 

$ apt list --upgradable

If You get a list You have to upgrade the rest of the packages manually starting with the first package in the list using the command

$ sudo apt-get install <package name>

do a new list, upgrade manually and then repeat until the list is empty and then 

$ sudo apt-get dist-upgrade

After You cleaned up with the commands below, do a sudo reboot and You will have a fresh Stretch OS. 

$ sudo apt-get autoremove
$ sudo apt-get clean

Remark !!

In one of mine upgrades the installation stopped with

Reading changelogs..
Extracting templates from packages: 100%

but after a good night sleep I did proceed with a hard reset, disconnect and reconnect the power to the RPi, logged on and checked the upgrade with 

$ cat /etc/os-release  

which reported "Raspbian GNU/Linux 9 (stretch)" so a lot must been going right, and therefore I continued with an update of the system with 

$ apt-get update
$ apt-get upgrade