I asked O2 the same question, and they told me I could not port my number from one O2 contract to another. The PAC code only works from one Network Supplier to another. The PAC code is an authorisation to the new Network to take over the Billing.
As has been stated in these forums by others, the only way to port your number from one O2 contract to another, is to port it first of all to a PAYG sim, then port it from the PAYG sim to the new contract. I confirmed this with the guys in a Tesco store yesterday.
yeah, i found a simular post as soon as i posted this one. thanks for replying though. hardly seems worth the effort as will be without own number for 7-14 days. might aswell get a new number afew weeks before the current one runs out and try not to forget to tell anyone the new number.