3GS and 4 sim cards

Just bought a Iphone 4 as my wife's upgrade. Planning on letting her have my 3GS and I take over the 4. So far so good! Aren't I kind?
Realise the sim cards are different sizes so have bought a sim card adapter to turn her micro sim into a full sized one to fit my 3GS and am going to cut mine down into a micro one to fit the 4.
Once her sim is activated through Orange UK, can I just put her sim in my 3GS and then put my cut down sim into the 4 and expect them both to work? Or do Orange do something with her sim so that it will not work in any other phone except the 4 it came with?
Beginning to panic a bit now as never thought of this before I ordered it!!
Thanks

You can do the SIM exchange thing. It's much easier though, just to call up the carrier and give them the number on the SIMs to change phone number the SIMs are registered with.

Thank you very much. I have just rung Orange and they are sending me a new micro sim and I have already ordered a micro sim adapter.
Appreciated.