Read read read
Its not just a matter of shoving some software on a usb stick.
You need to use a hardware flasher such as progskeet/teensy++ for NAND model FAT PS3's or an E3 flasher for NOR, and a FSM dongle.
NAND requires soldering, NOR uses a clip.
CECHGxx 0x05 SEM-001 2x 128MB NAND 40GB No Yes No No Yes 65nm 90nm 160/280 2 No 10/2007 1.94 1.90 or 2.30
Read the link above, and go from there. It covers everything about hardware downgrading.