I have tried researching but have not found any steps on how to do this. The closest I have read is the following old thread:
"Miui 8 : How To?"
https://xiaomi.eu/community/threads/miui-8-how-to.35415/#post-329906
For those who have successfully downgraded from Miui 8 to Miui 7 on the MiPad 1...