From what I understand it is not AMD putting artificial limit on upgrade/downgrade. 16M ROM is not enough to hold all support data for both new and old CPU simultaneously.
old BIOS + old CPU = boots OK
V
(flash new BIOS, install new CPU)
V
new BIOS + new CPU = boots OK
V
(install old CPU, for whatever reason)
V
new BIOS + old CPU = brick
So with the new BIOS, you will need a either working new CPU to boot & downgrade the BIOS, basically revert the upgrade process. Or just replace the physical BIOS chip with one holding old BIOS so that you can boot your old CPU again.
May be creating a backup BIOS chip, and flash the new BIOS on a blank chip just to be safe?