All depends on what type of PS3 you have (Fattie, Slim, Superslim) but heres the breakdown:
1. Fattie Console - Most if not all can be downgraded to the last hackable OFW which is 3.55
2. Slim Console - Most can be downgraded to the last hackable OFW which is 3.55 apart from if those where the minimal firmware version of your console (factory fitted FW) is 3.56+. There is a piece of software out there that you can run to check)
3. Superslim Console - You CAN NOT downgrade these consoles, if your looking to be able to use homebrew/backup managers on these.
If you fall under the 1 or 2 then your in luck as your console can be downgraded HOWEVER you can not do this just by using a software patch (i.e. install a CFW .PUP onto your console via a USB drive).
To downgrade your going to need a hardware flasher (either Progskeet if your console is a NAND or a E3 is your console is a NOR). Both of these required opening your console and stripping it down to the system-board also if you have a NAND console then some soldering is required.
If you are not comfortable with opening or soldering your system (if needed) then there are people who will do it for you for a charge.
yes that's true but as of now, downgrade is un-necessary step that you will be doing to migrate CFW from OFW. With latest NAND/NOR patching tools today, it will only patch the spkg and revokation keys to allow downgrades, this means once you patched those two you can install CFW straight from and 4.xx OFW without downgrading the console.
but still, you will need a console that is 3.56 OFW capable PS3 or lower to (Yeah not 3.55) to install CFW on your PS3.