Downgrade from V3.2 to previous V3.1 on Onyx Boox Max 3 (we have the V.3.1 upx file)

I will need to flash this tablet with V.3.1 firmware, so I need the procedure with step by step
details on how to do it.  We are aware that this is not recommended and should only be done by technically skilled persons or in Boox service centers. 

I know already the standard answer, which is:

"We are afraid that we are unable to provide a firmware downgrade instruction. In this case, you are suggested to submit bugs/software questions on Feedback via Settings/ Feedback on your device".

I already tried to submit the same request via the Feedback in Settings on the device, but 
the only reply I got was: 

"Android version restrictions cannot be downgraded by upgrading.
The firmware version needs to be returned to the factory to flash the machine. You can return to the factory according to the following process:  https://help.boox.com/hc/en-us/requests/new"

I have submitted the same request via that form, let's see what happens. 

I can understand that Onyx Boox don't want a full flashing procedure published 
and floating around on internet, this could cause harm if non-skilled people
tried it and ended up with bricked devices..., but at least they could send it 
non-public so we could flash the device back to V3.1 and taking the full responsibility 
for the process ourselves.