iphone 4s 16gb on o2 as new 3 weeks old , swap for galaxy s3 ?

I have a iPhone 4s 16gb black 3 weeks old ,had screensaver and case on it from new,, on O2 not sure if unlocked to any network,in as new condition still got over 11 month apple warranty still on it,, will swap for a Samsung galaxy s3 must be in same condition and work on O2/Tesco,,NO OTHER PHONES PLEASE and not for sale ,,thanks for looking