Can I switch to an iPhone before my contract is up with at&t?

Okay so one year ago I bought a blackberry torch through an at&t carrier. I'm on a family plan, and I have a data plan of $15/month. I signed a 2 year contract. I HATE this phone. I was recently given an iPhone 4 for free after that person upgraded to an iPhone 5. I was wondering if I can switch from my blackberry to this iPhone before my 2 year contract is up? I understand that an iPhone data plan costs more, and would be willing to upgrade. But will I be hit with charges for switching phones before the contract is up? I really don't want to wait until august 2013 to use this iPhone.

You can switch and as you said you will have to pay more. They may want you to extend your contract for another 2 yrs, too

I think in your case you probably can but your plan might change a bit. Normally you can't get the discounted phone price through them but you already have the phone now, you need to call them and discuss the options to see what your carrier allows.