perform SIM swap by following link
Hi. You can keep your current sim and request a new number. That way your goodybag and other account details will stay the same except for the phone number. You can request a new number in the "profile and settings" under the "my giffgaff" heading. Where your number shows, there is a change link at the end to that line.
You can swap the sim from here: http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084Incase you're unable to do a SIM swap from the above link then contact an Ask and Agent and give them the below details:- 19 digit of the SIM number which is at the back of the SIM card- Activation code of the spare SIM (they will swap it for You)