Seeing this question made me think about upgrading my iPhone 3G to iOS 4.x, if that's still possible.
I've deliberately kept my iPhone 3G on iOS version 3.1.3 because that's the final 3.x version; I tried 4.x when it first came out but decided to downgrade again (while I still could!) because I felt 4.x was too much of a burden on the little device.
However, these days apps are generally downward compatible only to iOS 4.3 so I've had to go without many apps. I'm considering an upgrade now to get access to "modern" apps but I'm not sure whether I can actually still upgrade my phone because 4.3 is no longer current.
– Is it possible, in 2013, to upgrade any device from 3.1.3 to 4.3?
I guess that if I upgrade, there will be no going back. I can't imagine a downgrade would still be possible.
I would also have to jailbreak it after the upgrade, only because I need the SIM unlock.
Best Answer
The final iOS rev for iPhone 3G is 4.2.1, and you could always save a backup before upgrading and restor to the 3.X version if 4.2.1 was too much of a hog.
Hope this helps!
Edit: This thread says its not even worth it. 4.2.1 is crippling to the device.