Go into the shop of whichever operator you're on and ask for a nano sim and they'll move it over and might also give you the adaptors for it as well so you won't need another sim.
... or you can buy a trio sim card from the shops (usually between free and £1), then contact your provider and have your number and credit transferred to that other sim card.
I might just wait till I have the phone and go in to the shop. My mobile number is the contact for the delivery and so I don't want my current sim to turn off early.
I've just had this exact situation, called my phone provider who are sending me a new nano sim, when I received it I just have to call them and they will transfer the number, because of the long weekend it should arrive Wednesday. All free.
Dave
I cut one down from full size to micro but didn't have the cutter to go next size down. And as it's free didn't see the need to pay for a new cutter.
Dave