This is not need anymore. I have successfully downgrade to nougat b130 from oreo beta. Indian version.
Just download the b130 dload firmware and follow to forcely flash upgrade with sdcard.
1. Extract the firmware and copy the dload folder in ntfs formatted sdcard.
2. Insert in ur phone.
3. Flash the ramdisk nocheck recovery by fastboot.then restart to system.
4. Thanks to @morph. Dial *#*#2846579#*#*. Then software upgrade. Then sdcard update. It will stuck at 99% and failed but Voila! U will successfully on nougat. The bootloader will be unlocked. Then normally upgrade to b150 by system update.
mine is stuck on 5% everytime