HP printers are programmed to automatically check for and install firmware updates. Even after a successful downgrade, the printer will try to update itself within days or weeks. You must block this.
However, you must be technically comfortable with USB connections, command prompts, and disabling network features. If you rely on HP Instant Ink or cloud printing, do downgrade. hp officejet pro 8620 firmware downgrade
Do not proceed without understanding these risks. An HP OfficeJet Pro 8620 firmware downgrade is not officially supported by HP. HP printers are programmed to automatically check for
A common method to bypass update restrictions involves manipulating the EWS: hp officejet pro 8620 firmware downgrade