When your vodafone contract expires tell them to send you a PAYG sim, this sim will obtain same number BUT will evidently be pay as you go. Once you've got an o2 contract they will supply you a sim with a fresh number. Keep this number till you fancy changing it from your voda. So by keeping your vodaphone sim on PAYG means it's still your possession and can do whatever you want with the number. So when the mood strikes and you fancy number transfer contact voda, get a pak sent out, inform o2 with PAK - be patient for a few days while transfers successful. Job done.