walk in to your networks mobile shop and say 'hey can you transfer my account from my sim to to microsim' they punch at the keyboard, ask you a couple of questions, like name address, stuff to make sure it's you, they scan a new sim card and hand it to you, it will work exactly the same as if you went in and you had lost your sim, in the process your old sim card will be canceled
Could always just cut it yourself.
O2 are giving away the Sim Adaptors as well, so you can turn your micro sim back into a mini sim.
Ive cut my Giffgaff sim card and it works perfectly in the adaptor.