Depends if leaving giffgaff is via here Pac code
Follow this link to leave giffgaff an keep your numberhttp://giffgaff.com/help/pac-code-request
an remember to login to your account .
Or dial 43431 and select option1 to get it your pac code will show here an is valid for 30days http://giffgaff.com/dashboard/
also remember if you used credit cards to buy goodybags go here http://giffgaff.com/profile/payment-details
and delete out card details an switch recurs of thanks if wanting to bring a number from another uk network you need to request a pac code from them an fill out this form Transfer http://giffgaff.com/support/transfer
date format is like this EG 21/10/2015