As iPhone developers, you have to test your software on every OS version.  This would be fine if there were a reasonably simple way to switch between iPhone OS versions.  As it turns out, there isn’t.  Upgrading is easy — but downgrading isn’t allowed by default within iTunes.

There is, however, a way to do it.  I figured it out the hard way, so I’m posting these instructions to help anyone else who might be in the same situation.  Partial credit is due to other tutorials that I borrowed information from.

If you’re getting iTunes Error 1600 and iTunes Error 20, this tutorial should help.

Disclaimers: Don’t do this with your personal phone, because you have to remove iTunes and completely reset the phone.  Restoring everything later is probably possible via backup, but make sure there’s nothing important on the phone before proceeding.  I don’t think this works on the 3GS phone.  But don’t even try it.

Downgrading from iPhone OS 3.0 to 2.2.1

1. Completely Uninstall iTunes 8.2 AND Apple Mobile Device Support
2. Install iTunes 7.7 (find it on an old version website).
3. Download the 2.2.1 (or 2.2) ipsw file for your phone.
4. Launch iTunes, connect to USB, and enter DFU mode (see below).
5. iTunes will mention restore mode.  Click OK and then Shift-click “Restore”
6. Choose the ipsw file you downloaded.  Wait for restore.
7. If this completes successfully, congratulations!  You’re done!  If you get Error 1013 (as I did), ignore and continue.
8. Hold the Power and Home button for 15 seconds, then press Power for 2 seconds to reset your phone.
9. You should now have the “Emergency Call” screen.  If your phone has service, it should activate itself.
10. If you’ve switched phones (to a 3GS for instance), you will need to either purchase phone service for it, or activate it via jailbreak.
11. Once activated, you’re ready to start testing!

Let us know if these instructions work for you!