So finally I have a phone running Android 6.
I followed the link I mentioned before. Only the sideload did not work so I used fastboot mode flash method instead:
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downloaded fp2-sibon-17.09.3-ota-userdebug.zip (on the phone)
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downloaded fp2-sibon-17.04.0-manual-userdebug.zip (on the laptop)
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downloaded fp2-sibon-17.09.3-manual-userdebug.zip (on the laptop)
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unzip the …manual …zips
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!!! backup backup backup !!! -> I used oandbackup for Apps+Data and TWRP Backup in addition
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flash fp2-sibon-17.09.3-ota-userdebug.zip via TWRP --> run into freeze reboot bug
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started the FP2 in Fastboot-Mode (Power-On + Vol-Down)
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run the ./flash.sh in the unzipped (17.04.0 manual) folder *
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rebooted phone (Android 5.1, stuck in boot-loop, optimizing apps)
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switched off by pulling out the battery
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started the FP2 in Fastboot-Mode (Power-On + Vol-Down)
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run the ./flash-for-unix.sh in the unzipped (17.09.3 manual) folder *
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rebooted
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phone started “normal” with Android 6 (but the apps lost the data)
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prepare oandbackup again and don’t forget to setup busybox correctly
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restore all the app data you need one by one
*flashing the manual-userdebug.zips via flash.sh script also means the related TWRP recovery will be flashed. As the device was encrypted I had to re-flash the latest twrp via fastboot like shown some posts before (otherwise decrypt is not possible). You can also edit the flash.sh script before executing it to exclude recovery flash.
A very long and annoying procedure. Hope the next major release upgrade will be more smooth.