(as @hirnsushi suggested - thx again) re-flashed CalyxOS with the device flasher → just following the normal install description: Install on Fairphone 4
I also locked the bootloader this time (more save for the un-expierienced Family member) - I was a bit unsure and then the device flasher stopped already so I did it via:
fastboot flashing get_unlock_ability
I got → (bootloader) get_unlock_ability: 1
and finally locked it via fastboot flashing lock
transferring the data (from the FP2 to the FP4) will probably be done via USB cable most (the messenger like WhatsApp and Signal have a nice Transfer to new Device functionality via WiFi - contacts are at google in their case anyway )
For Signal I just did it (CalyOS FP4 to my interims Phone FP3 DivestOS).
Works as described (my Signal container is 12GB btw.). But I also had the normal local backup at hand in case of issues. It took me 2 attempts to establish the Direct-WIFI between the devices but afterwards it went fine.
I though I made some screenshots, but I probably deleted them already again, but it is quite good described:
Just follow the Transfer from Android device variant. It creates a Wifi between the 2 devices and transfers the account including all messages (media stuff etc.). Final step is activating the Signal Account on the new phone, on the old one it is disabled then. Cannot be active on 2 phones (like Telegram or so)
Signal didn’t work directly and I finally used the backup method. The issue was that the versions were different (the one in the CalyxOS Repos was not up to date … witht the Wifi transfer method thes reason is not shown - error and it stops without further details → the backup method told me at the end and so I’m pretty sure that the wifi method would have worked as well if I knew that problem at the beginning)
Files (mostly pictures) I copied via USB cable (FP2 → Laptop → FP4)