Hp Officejet Pro 8620 Firmware Downgrade -

The utility will show an error like "Firmware older than current version cannot be installed." This means HP has locked the bootloader on your printer – you cannot downgrade. Some advanced users have attempted using a "downgrade package" via the printer's hidden Service Menu, but that is extremely risky and not recommended.

Disable automatic updates in the printer’s web interface immediately after downgrading, and block *.hp.com on your router for that printer’s MAC address. hp officejet pro 8620 firmware downgrade