I've got a Cubietruck and I just tried this: Replaced all occurrences of `jessie` with `stretch` under `/etc/apt`. All of them? Well, not all of them. There are some comments in `/etc/apt/conf.d/50unattended-upgrades` that mention `jessie`, and I found it's not good to replace those, they will come back with package upgrades.
It's great that Armbian has its own kernel for stretch, as well, so that one got upgraded, too. There was nothing I had to do specially (different from other Debian upgrades), and that was very very cool.