There have been a lot of rumors about the possibility to downgrade your phone 3GS from 4.0.2 to 4.0.1.

First the rumor that you can take the 4.0.2 IPSW and copy in the 4.0.1 modified contents is bogus. You will notice this only has worked for people who have set their HOSTS file to point to Sauriks (cydia) server. While the IPSW swap does nothing (so don’t do it), people who have refurbished 4.0.2 phones that have been previously jailbroken should be able to downgrade to a previous version as the SHSH blobs have been stored on the cydia servers.

Lucky for me we just downgraded a 4.0.2 to 4.0.1. Here is exactly what we did:
1) changed my hosts file to point to Sauriks server. Instructions here.
2) downloaded the 4.0.1 IPSW
3) restored to the 4.0.1 IPSW

To make sure what was going on we used TinyUmbrella to check for existing SHSH blobs for the phone. We found 4.0 and 4.0.1 blobs. This phone had been jailbroken /hacked by the previous owner and we were lucky enough to have the SHSH blobs on file.

Moral of the story: Always back up your SHSH blob using TinyUmbrella (even if you don’t want to jailbreak it now). You can also use TinyUmbrella to just check if your refurbished phone has previous blobs stored.

Special thanks to AnniePhish.

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