Only half of that made sense. You can upgrade to iOS 6.0 specifically if you have SHSH blobs for it. Or you can upgrade to iOS 6.1.2. If you're relying on a software unlock like UltraSn0w, you have to unlock it again after re-jailbreaking.
In addition... if you're relying on the ultrasn0w unlock you need to preserve your iPhone's Baseband by updating using a custom IPSW (iOS Firmware file.)