how to use the SD card for a SD copy is not a problem, I'm always doing that already, for copying all setupts from 1 SD of 1 Pi to the other and back and for backups. My question was about Dougie Lawson's post
You may be able to pull the *.deb from Stretch and install it on Jessie.
Pull the onboard 1.4.1-3 debs from http://mirrordirector.raspbian.org/rasp
You really need to look at what's preventing your upgrade, we'll be starting to replace Stretch with Buster in the fairly near future. Usually, the concerns that prevent an upgrade aren't realistic - build a test SDCard with the Piclone SD Card Copier (it's on the menu). Boot that then update every file in /etc/apt/source.list and /etc/apt/source.list.d/*.list with "Stretch" wherever you see "Jessie". Then run sudo apt-get update; sudo apt-get -y dist-upgrade; sudo reboot.
As already written several times, Stretch is no option (unless there was an upgrade from Noobs Jessie to Nobbs Stretch, by a simple 1-line command, also upgrading all Jessie settings and all installed Jessie tools and programs by just 1 key stroke, which obviously doesn't exist.)
But no idea what to do here for that was DL posted, the SD part for backup was not the question - that's clear of course - it's just about the rest to make "onboard" work on Jessie, for which I needed a ready-to-use install sh script.
Or, alternatively, a better virtual keyboard program which already exists for Jessie, i.e. better and more stable than matchbox, or kvkbd, or Florence.