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I would say go here to order yourself a new sim
and then here to transfer your service to it.
Unless anyone has an easier way online.
The quickest solution is if you have a vodafone shop near by, they will be able to do it there and then for you.
I did enquire about this recently, and the online SIM SWAP form definitely doesn't work for PAYG. See http://forum.vodafone.co.uk/t5/Pay-as-you-go-services/SIM-SWAP-problems/m-p/1751226#M34907 for details and alternatives - the 4th post in the thread.