At this very moment, yes, you can as Apple's servers are still bloated so they leave the option to upgrade open as they're focusing on alleviating the traffic from upgraders. You'll need to download the actual IPSW manually and restore it.
But a warning. You cannot use your iOS 7 backup after you've restored. If you have an older iOS 6 backup from before your iOS 7 update, you can restore using that iOS 6 backup.
This window of signing of iOS 6 is not a stable window, as it can close anytime without warning, so if you're looking to downgrade, it's best to do it ASAP rather than banking on a last minute downgrade.