This was all Solved and Talked about in March 2017 by
KennethMcNutt, ....He has it all Explained how to Down Grade... back then... I saved the Pages to my Drone folder on my Puter with a copy of all the different firmware versions...
Part of the Reason for Doing this was the Problems I was having with all of the Restrictions and RTH Problems, and Intermitent Loss of Signals... I replaced my EMU Board and Kept it's Early Firmware, and was at 1.5.7 ..... But this Spring, I flew my Drone for the 1st time in a yr, and the Camera got Hung Up on the Gimble Protector, and got over heated... It worked fine from Cold, through One Battery, then the FPV would go Blank, the rest of the Day... The Camera still recorded Pictures and Video.... You Just had No FPV... to Control the Shot....
I discovered there are 2 PC Boards in the Camera, that are the Brains for the Whole, P3 Standard Drone.... The Pro and Adv. are Completely Different, and use LIGHT BRIDGE Hardware and Software... So I know Nothing about them....The Front Board, inside the Camera, contains the CMOS, to take the video's, Memory Card Slot and Reader, and Possessor to control, to Control All of the Firmware on the Drone, and the 6 Axix Giro. to Control the Gimble and Camera Position...
This Link, Leads to a Treasure Trove of Wiki Info on How all the Boards and Systems Work and Tie Together....
DJI Hardware · o-gs/dji-firmware-tools Wiki · GitHub