If you require another size simcard and keep same giffgaff number then easiest option would be to walk into any market stall phone shop and have your simcard cut down to size so it fits the new phone. these shops also sell simcard adapters if you required a bigger simcard later.
You can perform a free simswap, simswap is a system which is used to replace a simcard with another one, you will keep your existing phone number and credits or goodybags, please note that other data (e.g. contacts & texts) are not transferred, so make sure to save them before you start:
Order free from this link once you are logged into your account:
Pick giffgaff simcard from most corner shops, local market stall phone shops