Are you already a giffgaff member and wish to replace the SIM? if so, Once the SIM arrives perform a SIM Swap. The SIM swap will transfer your mobile number and any active goodybag and/or airtime credit over to the new SIM.
You will notice the start of the process when you lose mobile service. The process ought to take no more than 30 minutes however, during busy periods can take up to 24 hours to complete. The SIM Swap can be performed anytime between 4.30am and 9.30pm.