Hi
You cannot swap thro o2 as you are past your 7 working day cooling off period.
If your not on a refresh tariff you could sell your current phone and use the funds to buy an Unlocked to all networks iPhone or one already on o2.
You would just need to speak to o2 to let them know your using an iPhone so your put in the right data tariff.
High street shops such as cex buy and sell Mobile phones.
You can use your contract sim in any such phone.
You could look at the refresh or speed to refresh tariffs but that's not remaining on the same contract.
http://www.o2.co.uk/mobile/refresh/frequent-questions/fast-trackhttp://www.o2.co.uk/mobile/refresh/frequent-questions