No you can update to 370 & still downgrade back to 355 you need to buy the E3 Flasher & something similar checkout your options.It will cost some money ,dont listen to somebody from a shop that says this.The proof is right here on HAX .Look at the front page http://www.ps3hax.net/2011/10/progsk...sion-released/
This is one way to do it .But would go with the E3 flasher b4 any others. Just my opinion.
Yes there is no hack or CFW 356 =Only downgrade firmware to go back to 355 then Jailbreak the 355
I know it sux ,to have to open the ps3 but there is no other way..
THIS SHOULD HELP U ->I would choose this -> ( http://www.ps3hax.net/?s=E3+flasher
By the way i do not know how old you are .If you are old enough to work you should be able to do this. If you are a kid & have no money you may need to ask for this from family .
Also may need a adult or somebody who knows what their doing to install E3 flasher.
They can do this for you so you don't screw it up..
This is the best advice i can give you.. Good luck..