Download the new version of my giffgaff and under one of the tabs it will say APN and giffgaff click that profile and it should change automatically.
This is the iphone settings
You can keep your sim and your number.
Take your sim to a local phone shop and get it cut down for a small fee. Alternatively report your sim lost and get a replacement - it will be a hybrid sim and will fit either size. Bear in mind though that with this option your sim wont work until the replacement is activated