I was never a global rom beforeSo, just download the miui_CEPHEUSGlobal_9.6.27_2b17a775d2_9.0.zip file and flash it with TWRP and you are back to global. What works on top of that (but you will loose all your data and apps, so make a backup to google and mi-cloud before) is to lock the phone again with fastboot. Then your phone is almost like it was supplied to you.
Correct so far@redenkri: As far as I know you can flash whatever ROM you want, China or Global, as far as you keep your bootloader UNLOCKED.
So never lock it again when you are flashing differents ROMs with TWRP!
You only can/should lock your bootloader again, when you decide to stay with the ROM channel which was originally intended for your device - in your case China ROM, as I understood your OP.
Someone please correct me if I'm wrong with my assumption.
Edit: Someone please correct me if I'm wrong with my assumption.