Why do you want to downgrade your BIOS ? Does the 1606 cause problems on your PC ??? I don't believe changing ONLY the Date would be enough but usually you can Force a Flash with the proper switch.
The date of original BIOS ver. 1606 is 09/03/2010 (present BIOS on M4A89GTD PRO) , the date of original BIOS ver. 1456 is 09/07/2010, the date of your SLIC 2.1 version BIOS is 05/14/2010! And there is no switch!
Those are Just the Dates they got added to the Website, NOT the actual Date the BIOS was Made. The 1606 (08/24/2010) is Newer than 1456 (05/14/2010). So I will NOT be attempting to change aspects of the BIOS one by one, just so you can Downgrade for NO Reason at all. Feel free to do that your self. Good Luck with that.
If that Really is an AMI Bios, Flash the BIOS and then Use AFUDOS or AFUWIN to make a back up and upload the Backup.ROM to be modified
I know that. The only problem is that Asus EZ Flash 2 doesn’t allow me to do that, I got following message: "Version in ROM file is older that existing board!" The same result I got with Bupdater V112! And BIOS ver. 1456 is newer than BIOS ver. 1606.
BIOS Version 1606 BIOS Date: 08/24/10 19:18:22 Bios Version 1456 BIOS Date: 05/04/10 15:06:16 BIOS Version 1606 is NEWER than Bios Version 1456, No matter what you seem to think. This Message tells you Exactly that: