Below the steps on how to downgrade your Lumia's Windows 10 to version 1703 or lower, required for 1703 procedure.

Please note: this is a last-resort procedure, since it will reset any content on your phone, so make sure you perform a WhatsApp OneDrive backup to be restored after.

  1. install Windows Device Recovery Tool on your PC
  2. connect the Lumia by USB and check it's detected by Windows Device Recovery Tool
  3. choose an older Windows version, like 10586.13169
  4. click on Reinstall software

When finished, install WhatsApp and restore it from OneDrive. Then proceed with 1703 procedure.