Install LuneOS for Tissot
Here's some information for anyone using a Mi A1, who has upgraded to Pie or further (which upgrades some low-level firmware and makes downgrading difficult):
I'm doing this from memory, so if I've made any errors, let me know.
This presumes that your bootloader is already unlocked. You also need a TWRP backup of your old Nougat installation, which must include the boot and EFS partitions. You'll probably want a backup of your current install too (including at least system, data, and boot), so you can restore your existing Android installation afterward.
You also need:
a copy of the Nougat factory OS and your current version of the factory OS to flash. a couple of versions of TWRP (either 3.1.1-0 or 3.1.1-1 plus either 3.2.3-1 or a newer version). These should be versions for the Mi A1 (codename "tissot"). Look around on XDA for them. the android tools installed on your laptop or computer (adb, fastboot, etc.)
put your device into developer mode and enable USB debugging connect the device to your computer by USB boot into fastboot (volume down + power) flash the Nougat factory OS boot into fastboot (volume down + power) fastboot boot "twrpfilename" [use twrp 3.1.1 here. LuneOS works best with it.] use twrp to flash boot and EFS (note: it is not recommended to install twrp to the Mi A1)
Then follow the LuneOS flashing guide here: https://forums.webosnation.com/luneo...ml#post3454659 You'll be starting on step #5. Note that twrp 3.1.1-1 works in place of 3.1.1-0.
connect the device to your computer by USB boot into fastboot (volume down + power) flash the current factory OS boot into fastboot (volume down + power) fastboot boot "twrpfilename" (I used twrp 3.2.3-1 here). use twrp to flash system, data, and boot (from your twrp backup of the current version). reboot into your regular installation (again, it is not recommended to install twrp to the Mi A1)