Not possible without a boot level exploit (preferably a hardware exploit). There are software exploits that you can use to downgrade an iOS version, but it's not worth getting them patched when they can possibly be used for a jailbreak in the future.The Pangu team stated a while back that they were working on a downgrading method, but why use the exploits for downgrading when you can save them to jailbreak the future iOS's, which is the only true reason why people want to downgrade.
