You won't mess up the CFW, you just have to find an external hard drive that can fit after taking the external cover off. B sure to get 1 that can b formatted to fat32 as well. I'm not sure witch ones are the ones that fit but you can google it. After putting the new hard drive in it will prompt you that the firmware on your hdd can not b run properly, just put your USB with your CFW of your choice back in and press n hold start select then it will format the hdd to fat32 and install the CFW. If you want to b sure to get it right with no mess ups b on 3.55 CFW or OFW. (If not there's downgrade firmwares for CFW where you can easily go from 4.xx CFW to 3.55) then when installing the firmware to the new hard drive put 3.55 ofw on, then after it sets up put on the CFW of your choice then your good to go with the bigger hdd
Note: you can use external hard drives for game backups with CFW