I have an iPhone 3GS. It runs well on iOS 4.2.x. I get prompted in iTunes to upgrade to 5.1, but I've been reluctant to do so, afraid it will make the phone sluggish and slow. The phone is not jailbroken and I don't want it to be. But I was wondering if it is possible if I upgraded to 5.1 and hated the performance of the phone, how hard is it to downgrade back to 4.x? Is it even possible? If so, how would I go about doing that?