If you have designs on downgrading back to an older iOS version you must save the SHSH Blobs (basically your iDevices digital fingerprint, and different for each iOS version) for each iDevice before upgrading, it's not possible to do this retrospectively. This allows you to create a "signed" .ipsw file for your individual device. Of course, if you upgrade to a Beta version Apple continue signing the older version until the full release of the new iOS so there's not a problem until that happens.