First determine on wich rom you are now.. my came on xiaomi.eu but older 8.7.5. rom on oreo so I needed to install older twrp (1102) to be able to flash to latest xiaomi.eu (pie).. but you need to have already downloaded twpr 1227 on hold.. so when you flash new xiaomi.eu don let it to reboot to system but again to twrp and then flash newer twrp 1227 (new xiaomi.eu will erase older twrp if you reboot to system).. from there is just weekly ota updates. And for that you need unlocked bootloader.
If you are migrating from .cn to .eu or stable it's I'm not much of a help.