I have a bananapi as well and like to move on to stretch.
I run the latest 4.xx kernel provided by regular updates.
So put together I need to:
* backup my current system of course
* alter all references in /etc/apt/... files from "jessie" to "stretch"
* manually purge "jessie" related armbian packages (see above) and install the proper "stretch" packages
* run the upgrade process
* reboot
* be lucky
or did you @Bernie_O or @gbi run into severe problems?
Thanks for any hints.
BR
Peter