Walking On The Wild Side
Upgrading the slaved RPi B+ to Stretch after changing /etc/apt/sources.list and then apt-get dist-upgrade.
I'm doing so while connected via screen over the USB console hardline.
Then again, that's being done over an SSH connection to the machine it is actually slaved to.
Thankfully this is all being done inside the LAN inside the house because I did and an apt-get -yd dist-upgrade to download all the necessary packages first.
As per the directions for upgrading, I did backup at least my home directory. I mounted on my laptop the home directory on the slaved RPi B+ and then ran tar cvnhpSf backup.tar [Mount Point] . Only a tarball under 9 gigabytes but that is worthwhile for my purposes.