You should be able to manualy update on your rpi1 b with a rpi2/3 image (image file in the update repository then reboot).
After second reboot, the rpi1 will hang but the same micro sd will then work fine on a rpi 3.
Information read on the third "additional testing note" on milhouse testbuilds thread here:
LibreELEC Testbuilds for RaspberryPi (Kodi 18.0)
Please notice "Note: Prior to migration I would recommend disabling all overclocks, as a Pi1 overclock may not work successfully on a Pi2, and vice versa."
But making a backup before is still a good idea.