Then why do people pay more money for phones that haven't been upgraded? I figured it was because you can't go backwards and the newest versions can't be unlocked.
When you try to install a firmware on your device (whether it's iOS 4.0 or iPhone OS 2.0), Apple signs the firmware using some combination of a unique identifier and other things to make sure you should be able to install it. When they release a new firmware, they typically stop signing the old firmware days or weeks afterwards, meaning you can't downgrade to it anymore. Also, there are checks on the device so even if you do manage to install it, you'll need to run certain commands to get it to boot up the first time.
The newer 3GS is harder to downgrade and you also can't downgrade the baseband (modem firmware) version, but you should be able to downgrade the actual operating system without any trouble.
Got it working. I followed the instructions and got stuck with error 1015.
I had to do two things.
1. Switch of your iPhone and put it in DFU mode with following instructions
- Click the Home + Power On button for 10 seconds exactly
- Release the power button and continue to click on Home
There are guides or official ways of downgrading?