Credits goes to airbender who started writing a very good tutorial. I could downgrade my P3P with ESC center board P01012.09 from 1.11 to 1.7 with success, so I just want to link to his thread.
ESC center board overview: https://github.com/mefistotelis/phantom-firmware-tools/wiki/P3X-ESC-center-board
This is important: The gimbal mainboard is getting hot during a few firmware flashes. The fan on the gimbal from P3P is not a great help. So it’s better to pretend overheating during multiple firmware flashes to simply take a temporary additional fan for cooling.
Firmware downgrade filenames:
To create the file, create a new file as a text file. Rename the file to one of the names noted above, do not include a file extension. The contents of the file shall also be blank (that is, no contents). Save the file. Copy to SD card with firmware you want to install and then power on the aircraft. ONLY USE THIS IF YOU KNOW WHAT YOU ARE DOING!