I don't know, seemed like there was too much conflicting information on the GG website if this was really supported and a few stories of the swap not working for some people so I thought I'd go down the blue peter route first.Out of interest why didn't you order a Free nano sim from the giffgaff site and use the sim swap feature? Once you have the nano sim you can do the switch online yourself in about 30 seconds.
http://community.giffgaff.com/t5/giffgaff-Top-Tips/How-to-use-SIM-Swap/td-p/6885194
If I botch cutting my current SIM then I'll have no option but to do the SIM swap and I'll let you know the result, but hopefully it won't come to that
Matt