Upgrade to iPhone 4S In Apple Store instead of O2 Store?

I have an iPhone 4 on O2 and I would like to upgrade to the iPhone 4S this Friday 14th, I want to stay with O2 but I want to buy my new iPhone at my local Apple Store since they will probably be better stocked than my local O2 store and I will have a better chance at getting one.

My question is, are Apple Stores able to cancel my current O2 contract (I've checked, I am elegable for an upgrade) and start a new one on my new iPhone 4S, or is that something I need to go to an O2 store for?

Thanks

Harry