Yes, I spoke to someone at 3 and all you do is get your new phone then when you want to port your number across call 333 (on the new phone) with your PAC code and they can port the number. I think I was told anytime up to 1/2 way through your contract. Might be worth asking on the chat thing on the store as well (especially because if you ask them a question they'll give you a code to get a free £20 accessories voucher with the phone
).