What you need to do is to perform a SIM Swap.
This allows you to transfer your giffgaff number to a brand new SIM card including existing goodybags and credits.
If you do not have one already then order a new SIM of the required size here:https://giffgaff.com/orders/mgm
Follow the steps here:http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
Bear in mind that:
You can only do a SIM Swap between 4:30am and 9:30pm.
Once started, SIM Swap is non-reversible.
If you are unable to see the first 6 numbers of the SIM serial number on a nano SIM - it will always be 894411 & must be entered along with the visible numbers on your SIM.
SIM activation code of existing SIM on your account can be found by entering the SIM serial number into the SIM Checker tool at: http://simchecker.giffgaffapps.com/
SIMs used in SIM swaps do not qualify and will not generate any bonuses or payback points even when originally linked to an affiliate SIM order.
It should only take about 10 minutes to swap, but that is not a guarantee, just based on experience.