iPhone 3GS users who had accidentally upgraded to iPhone OS 3.1 have reported that it was not possible to downgrade to iPhone OS 3.0 or iPhone OS 3.0.1 as Apple has stopped signing older iPhone firmware files.

iPhone Dev Team claim that some users (not all) who have accidentally upgraded to iPhone OS 3.1 might be able to re-jailbreak their iPhone 3GS.

Warning: Please note that this is meant only for iPhone 3GS users who have accidentally upgraded to iPhone OS 3.1.

iPhone Dev Team is still warning iPhone users who have jailbroken or unlocked their iPhone to avoid upgrading to iPhone OS 3.1.

According to iPhone Dev Team:

If you want to come back to the world of the jailbreak (but NOT the sim unlock, sorry!) then saurik’s new “on file” server may be able to help.

He’s got all the details in a new article so do check it out.

They have also pointed out that:

there’s no guarantee that your 3.0 signed files were captured by saurik in time.

So if you’re not able to re-jailbreak your iPhone 3GS then it’s probably because your iPhone OS 3.0 signed files were not captured by Saurik in time.

They have provided additional information to users who haven’t upgraded to iPhone OS 3.1 and waiting for the Dev Team to release their PwnageTool for iPhone OS 3.1:

NOTE #1: the carrier sim unlock is a different story. Jailbreaking and unlocking have two different security mechanisms, and if you update your 3GS (or 3G) to 3.1, you will lose your carrier unlock, possibly forever. Even if you downgrade from 3.1 to 3.0, you will have lost your carrier unlock. So if you think you’ll ever want to carrier unlock your 3G or 3GS (or maybe give it away or resell it later as an unlockable iPhone), then please stay clear of all official Apple IPSWs. You’ll soon be able to create custom 3.1 IPSWs using PwnageTool that let you pre-hack your 3.1 update in a way that preserves the carrier unlock.

NOTE #2: The custom IPSW flow using PwnageTool also ensures that even if Apple fixes all the iBoot holes, you’ll still be able to retain your jailbreak through later updates. That’s because a jailbroken iPhone will happily accept a custom (pre-jailbroken) firmware update even though it’s not blessed with Apples signatures. This is the “once jailbroken, always jailbroken” approach. It’s very powerful, but it requires you to only update to pre-hacked IPSWs.

NOTE #3: None of this applies if you have an iPhone 2G, iPod touch 1G, or iPod touch 2G. The iPhone 3G is also unaffected by Apple’s signing process for the jailbreak, but it is susceptible to permanent loss of the carrier unlock as mentioned in note #1.

As always, please let us know how it goes.