There isn't any downgrading method (that we know of), yet. You can only "rollback" from beta versions of 5.50 (there's an option in the menu for that). It's an official method, from Sony, to let people go back to the previous stable version if they encounter problems and they're unhappy with the beta.
All right? There isn't any "HDD thingy which allowed downgrading..." There never was. If you see a website promising you that you can downgrade, it's 100% fake. They want you to complete surveys, download a bunch of crap (viruses) and possibly even brick your console, because hey, why not.