Go on ebay look for Orange USIM they are about a fiver now i think new and sealed, was a tenner back in the day i got one.
When it arrives, ring CS and tell them you have aqquired a 3g sim and wish to port your number to the new sim, give htem the new sim card "number" on the packet and it should be doable. I got a really nice gal who transferred it all for me and send a hit to the other sim to kill it of. Just proves its possible, so dont let them tell you otherwise!
good luck mate.