Can I downgrade my PSP from 6.00 OFW?

Can I downgrade my PSP from 6.00 OFW?

I have a PSP 1000 (Phat) and for some reason it was updated to 6.00 Official Firmware (OFW).
Origninally I had M33 (Custom Firmware) etc. But lost it in the update.
Is their any way to downgrade it without the need for say a Pandora Battery or Magic Memory Stick to get it back to the Custom Firmware?

If worse comes to worse I will have to buy those things to do it, just needed to check if there was a way of doing it for nothing.

Thanks in advance for any answers.