I have to get a new SIM card to get a new phone?

My current phone is the Sony Ericsson W910i, and I'm on O2 pay as you go. I I want to start or the blackberry curve 8900 or the Sony Ericsson Xperia X1 – but I really do not want to have to change SIM cards, as I'll have to change my number and I do not want to do that. Is there any way to change the contract offers (for example, to pay a monthly contract), without changing my number? Or is it possible to obtain any phone sim free and unlocked – but my sim card work with these phones?

release is a good idea. Here in India About 70% O2 and BlackBerry are released. Unblocking means defining Handset free to accept any Sim Card Service. But even the release Inquiring Person The idea of the Cell would be good