You ask for your PAC first from your current network, and when you receive it, you have around 30 days to use it. Usually it's part of the sign up process with the new network. It'll take a few days for the transfer to happen so you'll be stuck with the new number temporarily for a while, but on the day of the transfer, once you notice no signal, reboot the phone and your old number should be properly assigned.
Your existing number will remain active until another network ports over the number using the PAC. Been a long time since i ported my number, but think it's 1 month / 30 days which are given for a PAC until a new request needs to be raised.When I get my pac, will my existing number not work? Also I get billed on 1st of the month, I guess I'll have to pay £10 to both no ways around it?