Before you downgrade, you must stop the printer from automatically re-updating itself.
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🚀 Once the downgrade is successful, never click "Yes" on your computer when a popup asks to update your printer software. If you'd like to proceed, I can help you: hp officejet pro 8620 firmware downgrade
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. Before you downgrade, you must stop the printer