Even if a downgrade is blocked, you can sometimes force a or an unsigned older version via emergency recovery:
— it may trigger auto-updates again.
(as Administrator):
Plug the printer directly into a wall outlet, not a loose extension cord, to prevent power loss during the flashing process.
Proceed at your own risk. The following methods have been documented by the community. They are not guaranteed to work for all firmware versions, and HP has made it increasingly difficult to perform downgrades over the years.
