From the research that I've been doing myself, you can only downgrade with a flasher. And that is considering if your PS3 is downgradable to ofw 3.55 or lower. If it's a phat PS3, then they should be all good to go for the e3. If it's a slim, you would have to check the serial number on the back and see. There is a picture that is floating around the internet.
So either you get the flasher or you'll have to get someone to downgrade for you.