The purpose of a PAC (Porting Authorisation Code) is to allow you to transfer your number to another network.You get the PAC from the network operator you want to leave, and then pass it to the network operator you want to move to. The PAC is usually valid for 30 days after which time it expires.
PAC codes can be obtained from this link http://giffgaff.com/help/pac-code-request or dialing 43431 When your SIM card is deactivated or you port out, it does not affect your giffgaff account so you can still be part of the Community. Your giffgaff account will act in the same way as it did before you activated a SIM. Any credit/goodybags/gigabags available on your SIM card at the time of deactivation is lost If you activate a new SIM, you'll be able to register it with your existing giffgaff account during the activation process
You cannot do both and retain your number with giffgaff. If you obtain a pac code and port your existing number to another network then in order to stay with giffgaff you will have to activate another giff gaff sim which will give you another giffgaff number and therefore have another giffgaff account.