Swapping iPhone 4 / iPhone 4S

My spouse has an iPhone 4S and I have an iPhone 4 (due to when we purchased our phones0. She is a phone luddite and refuses to use Siri. With the news iOS 6 will not support Siri on iPhone 4, I'm thinking of exchanging phones. I know I can swap SIM cards between the phones, but will she and I have to both reload our phones due to the serial numbers as recognized by iTunes? Or can backups (for each of our phone IDs on iTunes) be enough to restore the "desktop", but on a different physical phone/