same here...
really need to downgrade ? in order to get ota ?

1. The 2 zip files only can be install via twrp. Means you need to unlocked your bootloader and install twrp(warranty will be void).

2. Another method is to downgrade your firmware to L29C636B211 via dload and hope you will get B521 OTA update. This method is without unlocked bootloader and twrp.

Method no. 2

** Make sure you backup your files before proceed.

1. Download L29C636B211 "update.zip" firmware.
2. Extract update.zip and copy update.app to dload folder at your sdcard/usb otg. If dload folder doesn't exist. Create the folder.
3. Shutdown your phone
4. Insert/plugin sdcard/usb otg
5. Press and hold volume up + down + power
6. Release it once huawei logo appear.
7. Wait until complete and let it load. ***
8. Once you're in. Go to Setting -> Software update.
9. You will get B521 OTA update.  (I assumed you have registered as beta tester.)

*** If is doesn't load. You need to wipe the cache by entering erecovery. Press and hold volume up + power. Release it once you see huawei logo.

If you don't mind to void your phone warranty then only I will guide you using the method no. 1

Good luck.

I downgrade from 321 to 211 on the 28th Dec and I get the B521 on 29th...

Yah, backup data with Hi Suite, manually copy DCIM, photos folder and whatsapp media folder

downgrade to B211, wipe cache, reset factory and restore all app and media. all good. even with app arrangement 90% back to normal.

Hvn downgrade it yet. Still waiting if they will provide me the beta firmware

hvn do it, if wipe data i wont do. rather wait for official release