Did you block your old SIM card and order a replacement SIM at the same time?
If not then the new giffgaff SIM card will just need to be activated on your account after logging in https://giffgaff.com/activate
On successful activation, your mobile number along with the goodybag and airtime balance will transfer over to the new SIM.
If you ordered a replacement SIM while blocking the old SIm then only the replacement can be activated, no other SIm will work!
The SIM should get there in 2-3 business days though.