Just had a conversation with o2:
Tim: Hi there. My name's Tim I am an O2 advisor, may I take your name please?
You: Hi, my name's Chris.
Tim: Hello Chris, How are you today?
You: Yeah, pretty good thanks. yourself?
Tim: Im great thanks.
Tim: Welcome to O2. What brings you here today?
You: Yeah, i'm just thinking about buying the iPhone simplicity 12 month for £15 tariff
Tim: Fantastic.
Tim: Would you like me to send you the link to buy this deal Chris?
You: I'll be wanting to whack the SIM into my iPhone 4 when it arrives... Will I be able to swap the contract over to a microsim to do this?
Tim: Sure, once you have the sim and want to change it just call 202 free form the sim then we can get you a micro sim sent out.
You: That's awesome. That's just what I wanted to know

Thanks a lot. Now I can't wait til the 24th. Think you could send me a microsim ahead of time?
Tim: Im afraid i can't the only way to acutely get one is to pre register however if you do this you only may get a sim send out.
Tim: Plus it will not have the contract tariff on.
Tim: But if you order that sim then call us once you need the micro sim, this is the best way to get one.
So there you go.