I have successful achieve the downgrade of my p3 advanced from 1.8 to 1.7
The procedure is..
Format the SD card (FAT)
Copy the FW 1.7
Insert the card and power the drone. No RC connected.
Leave for awhile.....nothing happens and it is ok.
Power off
Then before power on again, press and hold the rc link on your aircraft
Power on pressing the rc link, and hold until first four beeps. Release the rc link
You will see the the downgrade will start and all the beeps as usual.