i did this the other day mate i upgraded my 120gbcfw to 320gb from my ofw its easy!
simply install 3.55cfw or ofw on your jailbreak then remove any disc or usb connected swap hard drives and then when you switch it on it should promt you to install a firmware of 3.55 or higher
remember it formats so you wont have anything on the hardrive and if it dont work or you messed something up you can put your old one back in
any problems skype me: MoWahidd