Welcome to the community
If you want to change your number then there is no need to change the SIM.
For changing your number you must have £10 credit in your account.
There are no charges of changing the number on giffgaff.
In case of changing the number all of your credit, goodybag and allowances will remain same.
For getting more guideline please follow the linkhttp://community.giffgaff.com/t5/Solve-your-questions/Can-I-change-my-Mobile-Number/ta-p/3488137
The agents will contact you for getting details of this process.
After changing the number you will need to top-up.