As promised the iPhone Dev Team has released a new version of the redsn0w jailbreak utility with one major new feature: You can now use redsn0w to downgrade any current iOS device to an earlier version of Apple’s operating system.
There’s a new “restore” option in redsn0w 0.9.11b1 and later under the “Extras -> Even More” menu.
In order for the restore function to work you need to have backed up your personalized SHSH blobs for your device using the Cydia store’s TSS@Home function or the TinyUmbrella utility.
In other words, if you think you might ever want to downgrade, now is a pretty good time to backup your blobs.
The new downgrade function will update your device’s baseband, so if you have a carrier unlocked iPhone, you should avoid this tool.