Iphone 4s sim card transfer?

I'm currently on an orange contract and using a Samsung galaxy ace. I'm not due an upgrade for another 10/11 months but was wondering if it were possible for me to buy the iphone 4s and a micro sim and ask orange to transfer my contract to the iphone? Would this be 'aloud' or do i just need to wait for my next upgrade is availiable?

No problem, if the Sim cards habe the same size.

wkr p