Yes that's fine. If you've lost it in your house and there's a chance you'll find it
Perform a SIM swap (see below)
If you've lost it outside and there's a chance someone will use it
Report it lostHttp://www.giffgaff.com/support/lost
(sim order and sim swap procedure)
Order a sim from your spread pageHttp://j.mp/order_sim
When the sim arrives
Click here and log inHttp://j.mp/swapsim
Put the 6 digit code in from your new sim And click continue
On the next page it'll show the 6 didgit activation codes and sim serial numbers of your current and new sim cards
Click where it says...
"Yes I want to replace my SIM" ( on mobile browsers don't forget to scroll down to see)
On the next page click "yes I'm sure"
Your new sim will hopefully be active within 30 minutes And your old sim will be disabled ( if the new sim dosnt appear active after 30 minutes try turning the phone off and on