Just order yourself a sim from here- www.giffgaff.com/orders/mgm
Then do Sim Swap: http://bit.ly/giffgaffswap
This will keep your number and any credit etc you may have
Or if you don't already have a giffgaff account with an activated sim card, then just order from someones banner to get £5 free credit with the sim
Standard giffgaff sims these days convert to micro sims by popping out the middle. Have a close look at your sim -- can you see lasered lines around the contacts? If so, just give the middle a push and you will have a micro sim.
Otherwise you can order a sim through your Spread giffgaff page and do a sim swap, as others have said.