see this guide to 4g http://community.giffgaff.com/t5/Using-giffgaff/Guide-to-4G/ta-p/12134248#
then check Frequently Asked Questions
more info here http://community.giffgaff.com/t5/Tips-Guides/What-s-new-with-4G-How-to-help/m-p/15344289/highlight/false#M119151
follow these 4 easy steps https://giffgaff.com/4g
order another sim here: https://giffgaff.com/orders/mgm
you may need to back up your contacts from your old sim first
For sim swap, log into here https://giffgaff.com/activate
Put the 6 digit code in from your new sim, And click continue,
On the next page it will show the 6 digit activation codes and sim serial numbers of your current and new sim cards.
Click where it says... "Yes I want to replace my SIM"
On the next page click "yes i am sure"
swap will not take long, and you will be up and running with your new sim/credit/goodybag and current number in no time at all