Downgrading directly from 4.89 OFW to 4.88 OFW is generally through software alone because the official system will block any "update" that has a lower version number than what is currently installed.
As the installation completed and the system rebooted for the final time, Silas checked the System Information System Software: Version 4.88 how to downgrade ps3 firmware 4.89 to 4.88
Follow the prompts to patch your NOR/NAND Flash memory. This "tricks" the console into accepting firmware that isn't digitally signed by Sony for that specific version. Step 3: Installing 4.88 Custom Firmware (CFW) Downgrading directly from 4