You can now activate a new sim if you loose one or it is damaged, or you need a micro or nano sim, by doing a sim swap.
Log out of gg, then click activate a sim, enter the 6 digit code, then select i have a gg account, then click sim swap. All your account details, credit and goodybags and number will be swapped to the new sim.
Please note, that sim swaps stop between 9.30pm and 4.30am
Or 2 Report your SIM or phone as lost / stolen http://giffgaff.com/support/lost and you will get a replacement SIM (this will be hybrid SIM which is a normal sized SIM that can be popped out to microSIM format). Further you will keep your number and credits.The problem is that once you report your sim lost or stolen, your account is blocked until you activate your replacement sim.
Note that for security your replacment SIM will be blocked when it arrives, activating it removes the security block, but sometimes you will need to ask an agent to get it unblocked but this does not require a further topup to do.
If you have a spare un activated Giffgaff sim you can do it straightaway. If not just order a sim from your dashboard or from any banner on this forum. Giffgaff now send out hybrid sims, which contain both a standard and mirco sim that you can pop out.