We want to test our iOS app on a few devices (iPhone 3G and 4 - or equivalent iPod Touch), but we also need to ensure it will run on the major releases of iOS, maybe 3.0.x, 3.1.x, 4.0.x, etc - I'm a little rusty on the version numbers and who got free upgrades to what.
I've read Is there a way to downgrade a 2nd gen iPod touch from iOS 4 back to 3.1.3? but I'm still unclear whether or not we can just buy a single iPod Touch and downgrade and upgrade as necessary, or if we'd have to buy an iPod Touch per iOS version we want to test.
So, what are the specific restrictions placed on downgrading? Can we just get our devices into DFU mode and apply whatever version we want? I don't want to end up buying a lot of devices if we only need one each.
We don't need any phone-only hardware such as the microphone or sending SMSs, so that's not a factor.