Just upgraded this server from Debian Jessie, to Debian Stretch. Went very easily, except for a problem with firewalld. Reinstalling dpkg, allowed it to sort out the problem with finishing the upgrade.

Change sources.list to stretch repos.
Then.

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade
sudo reboot
sudo apt-get install --reinstall dpkg
sudo apt-get dist-upgrade