Samsung Galaxy S3 white boxed sell or swop for an iPhone 4S +

I recently recived from 02 a Samsung Galaxy S3 white mobile phone (2 months old) but after trying it I have decided I would prefer an iPhone 4S This phone is on a contract but I have spoken to o2 who say that it will be okay to either sell the Galaxy and buy an Iphone or to swop for an iPhone and the sim would just need put into my new phone Phone is 16gb with a 32gb memory card installed so total memory 48gb Please email if you are interested