You can't downgrade/upgrade the OS to a specific version of an update after a fortnight or thereabouts. That's an artificial constraint applied by Apple. It's not even possible to do so offline with iTunes even if one has the IPSWs. If you check the /r/jailbreak subreddit, talented coders have to hack the SEP and build complicated, low-level-interacting software like futurestore in order to perform a semi-successful downgrade/upgrade.