upgrading 3GS to iPhone 5

I have an iPhone 3GS, which I'd like to upgrade to a 5.  I don't want to buy via my network provider, as they'll make me take out a new long-term contract, and I'm quite happy on my rolling one-month contract and don't want to change, as it would be more expensive to do so.

If I buy the handset direct from Apple, I understand that the SIM for the 5 is different from the one in the 3GS.  So is there any way I can buy from Apple, then keep my existing phone number, as obviously a new SIM will be required as part of the change?