Sure they can. The same way they just changed the private key. As long as the old public key can still be accessed by old games and older firmwares then it's fine. The reason Sony will want to change it is to protect newer games from piracy and force people to buy a new console or update from CFW to OFW if they can't afford a new console.
I will be very surprised if CFW guys bother with anything beyond 3.55 now. The fact that they have private key access is awesome for them and any CFW beyond 3.55 will just use the old private key anyway. Honestly it is the best of both worlds for them. They get to play games if they get ripped and patched with the old private key and they get their homebrew programs as well. It will be once Sony start using 3.60 firmware features that it becomes difficult and people will have to buy new consoles or hope for a new CFW with access to the new private key.
Man, I dont think you're right here. Check this:
http://www.next-gen.biz/news/ps3-356-firmware-%E2%80%9Ca-minor-drawback%E2%80%9D-to-hackers