I finally found some time to try this image on a donated FP1 that has a broken USB port.
The instructions say that one needs to restore the IMEIs after flashing the image.
But for me the correct IMEIs were shown when entering the engineering mode > CDS Info > Radio Info 
Looks like restoring them isn’t necessary in this case.
BTW, originally I was triggered by a Tweet by somebody who said they weren’t able to run Twitter on their FP1 anymore as apparently they dropped support for the installed version of the Twitter app and newer versions are no longer compatible with Android 4.2.2.
I did some more testing on the phone with the broken USB port before trying to upgrade my own FP1 that I use as backup (e.g. when going to festivals). I found that none of my car-/ride-sharing apps (except one with rather limited service) are compatible with 4.4.4. So there goes the possibility to use the FP1 as backup 
I tried DriveNow, Moia, ShareNow, Sixt, none of them could be installed and the old versions I have installed right now are not working anymore.
Android 4.4 is just super old by now and many apps don’t support that version anymore because the market share is just extremely small by now (especially in European countries, which most apps we use target) - and targeting e.g. Android 4.4 and lower makes the job as an app developer more difficult.
Yes, I know. That post was mainly intended as information for other people who might want to try out 4.4.4 for the first time hoping it would address their app incompatibility issues (it still does for Twitter).
That is so sad and true, @Ingo and @z3ntu … I know wanted to upgrade my daily-use FP1 (4.2.2), but I don’t dare it because I wouldn’t be able to reinstall all my apps which works still. I have already to sacrifice a lot of them, e.g. BlablaCar app and other stuff. Very annoying. So in fact the FP1 is dead now, you can’t upgrade the OS and can’t get any more app updates. In a few month there won’t be any working apps anymore, I guess. So I would need another smartphone, but this is another topic…
Well, now I am trying to update my old FP1 nevertheless. But I am stumbling on a problem: when I type su, it says permission denied… What can I do there? How can I give superuser-rights to the terminal app (if this is the solution)?
Type root or sudo, but usually this is not needed. Are you following the installation instructions?
yes. I typed su. sudo wasn’t accepted by the shell… root neither.
Which step are you at and why are you doing anything in your phone’s terminal app?
@hnms had the same problem last year and was able to solve it using a different terminal app (“ConnectBot”).
Is Root access enabled in the developer options / in the the Superuser menu in the settings?
Thanks for your suggestions. In fact, I changed the terminal app, to the same that @urs_lesse referred to in his solution, and now it works! I am so excited, I will do it right now…!
Well, after a bumpy upgrade (first it didn’t seem to work), it’s up and running now!
So now I am trying to get back all my stuff. But Internet doesn’t work… any ideas???
Once I will have it working, I will install f-droid and titanium bckp, and restore my apps… does this sound like a good plan? And get xposed from somewhere…
Thanks for the work, @z3ntu!
So somehow by re-installing AFWall+ and rebooting two times, internet works again! 
But after trying to install Xposed, I went into a bootloop… I made a factory reset… again! Anyone knows how to install Xposed and Gravitybox without bootloop? That would be awesome!
Thanks to all you guys here!
I understood 10% … I have no idea how to make a back up too… sorry about
Hey Pablo,
you can use either the build-in backup tool (I don’t know about his quality, but I used it), or install Titanium Backup, and do a full backup. Afterwards you can follow the steps in the guide of Urs. Hope this helps!
Cheers!
Hello Fabian,
if I can’t let my FP1 work on my pc (another opportunity maybe available) I’ll do the full back up and repeat the useful advice that everyone gave me on this chat. Thanks
Do you mean you have problems to connect your FP1 to your computer?
Hello BrFabian, yes I wanna try to connect my FP1 to my computer and go inside booking.com extranet from my mobile. That’s can maybe be the easiest way to don’t update all, in case I can’t I’ll do the suggestion that you gave me. Thanks and seems you have a FP1 and same problems but with more knoledges then me