I assume you're after taking your normal sized sim card out of your old phone and using it in the new phone, which takes a smaller sized sim card ??
You'll need a sim cutter - do a search on Ebay - they're only a few quid :)
well I just ported my number from an O2 phone to an O2 phone for the 4th time!! twice from PAYG to PAYG, the next time from PAYG to pay monthly and this last time back from pay monthly to PAYG.