For those wondering how to flash ROMs, which ROM to flash, it's quite easy once you've figured it out. The instructions aren't very clear (they seem to get confused between fashboot/download mode).
Even with a locked bootloader, you can flash between global/china, stable/developer roms with no problem.
Go here:
http://en.miui.com/a-234.html
Click "Fastboot Update".
Download the MIUI ROM Flashing Tool.
Download the relevant ROM, for example "Redmi Note 3 Qualcomm Latest Global Stable Version Fastboot File Download"
Power off your phone. Hold Power and Volume Up to enter recovery. Tap 'Download'.
Your screen will go blank, but don't worry. Connect it to your PC.
Open the MIUI ROM Flashing Tool. You need to extract the Fastboot ROM you downloaded earlier to a folder, and copy and paste the location into the blue box shown in Step 5.
Your phone should be showing as COM 0 or 1. Click Flash and when it's completed you just need to unplug the phone, hold the power button and it should reboot.
Once you're on the official stable/developer ROM, you can update OTA, through Mi PC Suite or by placing the ROM on the phone and using the updater app.