When I first joined Giffgaff and changed my phone the lady at the shop where I brought my new phone actually cut the sim card down to size with a special cutter but advised me at the time, I take no repsonsibilty if it fails.
She went ahead and cut it down and it was fine.
That was a few years ago and since then I have done two sim swaps through the GIffgaff sim swop link and both worked great with no trouble.
But be advised as it only transfers your credit and phone number.
Contacts you have to input again manually.
Just thought I would mention it.