Yes, you can install an older custom ROM if you unlock your bootloader
https://xiaomi.eu/community/threads/unlock_hyperos_-global-devices.71752/
After that, you should be able to flash an older official ROM if you want
@fcarrico The instructions are in the first post of the threads.
https://xiaomi.eu/community/threads/hyperos-1-0-stable-release.71170/
https://xiaomi.eu/community/threads/hyperos-2-0-stable-release.74372/
Yes, if they "open sourced" their modifications on closed source applications, they would probably have issues with Xiaomi really fast (let alone the security issues with the community publishing updates).
I am not sure what you are looking for, but you can keep track of the Xiaomi EU module releases here : https://xiaomi.eu/community/threads/xiaomieu-module-app.70890/
You say that your phone is not a Chinese version, and not a global one. Which one is it ?
Did you check which region your account is attached to on https://account.xiaomi.com/ ? Does it match the region your phone is coming from ?
Flashing the ROM itself is not hard, it is just a few command lines (for the EU ROM), or a few clicks (for the official ROM with MiFlash).
But the locked bootloader prevent you from installing unofficial ROMs and I am pretty sure you cannot install an older official ROM over a newer one. So to...
You have to keep in mind it is not an easy task, it would probably be a LOT simpler to buy a battery or to plug the phone. You may brick your phone or put it in a state where you are not able to finish the process.
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Backup your data, you will lose them in the process.
You would need to...