Switch SIM cards
I attempted to upgrade to ISO 9 and now phone is locked on slide to upgrade screen. Can I take out the SIM card on my iPhone 4s and put it into my old iPhone 4 and will it work?
Yes, so long as the iPhone 4 is on the same carrier as the 4S, it will work.
To fix the "Slide To Upgrade" bug, see Apple's support instructions.