Just wanted to inform that the method 1 for going back to FP OS does not work, at least from version 16.07.1 of Open OS to 1.5.1 of FP OS.
It downloads the file and asks for restart, but then restarts in recovery mode and nothing happens.
Closed, because these are no longer relevant OS versions.
If anyone runs into troubles going back using version 1, just use version 2.