I'm very prowed to say that I succesfully downgraded my phone.
I nearly tried everything and nothing worked, always blackscreen
when booting with the old gb bootloader.
The probleme is that some p920s need a special
x-loader and u-boot file which nobody backed up. I just
got them and now restoring to gb will work for everyone.
Use this files too if you've downgraded from su760 ics port
and your having problems with network.
How to make your phone work on gb again:
1. Download this and extract:
2. flash complere bootv21e when you are on ics
3. then go into fastboot mode with wkparks omap tool
4. open cmd and go into the directory with the downloaded files
fastboot flash u-boot u-boot.img
fastboot flash x-loader x-loader.img
6. fix your imei
hope it works for you for me worked nothing else