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.