Yes, you can!
You can use edl mode to downgrade your phone:
- Put your phone in fastboot and type fastboot oem edl
- Your phone will then enter in edl mode(kind of emergency recovery state), it will be a black screen, don't be afraid!
- In edl mode you can use an older version of miflash, in other words, miflash 2015.10.28.0 with a x64 computer\OS to flash(even with a locked bootloader, it does not matter) an older version of miui, in this case libra_images_6.1.7_20151221.0000.11_5.1_cn_b09dac70a0.
- After the flashing is done, you need to reboot your phone and you do this by pressing the power button for 8 seconds, you will need to wait for your phone to boot up to 6.1.7, after it boots up, you just need to power off you phone and put it in fastboot again and type fastboot oem unlock.
- Congratulations, you now have unlocked your phone yourself and as a bonus you just gave xiaomi the middle finger for their s**t unlock policy and system. Clap, clap, clap, clap...
- Now you can flash twrp or flash official xiaomi roms.
PS: I'm not responsible for any mistakes or damages that you can cause to your phone! Do this at your on risk!