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

I have a iPhone 4s 16gb black 3 weeks old 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