If you already have a giffgaff number, then you can swap the new sim to get your number back from here:http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
Once you do a SIM swap all the account details of your old SIM will be automatically transferred on the new SIM along with your goodybag and it take up to 24 hours.
Incase you're unable to do a SIM swap from the above link then contact an Ask and Agent from here:https://giffgaff.com/support/ask/activations
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)