Can i swap my contract blackberry for an iphone 4?

Ok I have a blackberry 9900 on an 02 contract that still has about 18 months left and i hate the blackberry i was wondering could I sell my blackberry and buy an iphone 4 and put my contract sim in it only problems i have is that the iphone has a different sim and is it illegal would i get in trouble from o2? I would obv still poy the contract

BlackBerry sims only work in BlackBerry's, so its not going to work. You have a contract so you have abide by it, although you CAN cancel it but you still have to pay for it.

Yes if you blackberry isn't a micro sim then simply cut it up yes it sounds weird make sure you only don't cut into the gold
http://m.engadget.com/2010/05/06/how-to-create-your-own-micro-sim-card-using-a-chef-knife-and-som/
and I'm don't think it's illegal as you are still paying your contract and its a free phone it makes no different as long as they get their money each month they don't care
You have a good idea go for it