O2 Replacement 3g Sim Card

Hi,

Does anyone know how i would go about registering or connecting a o2 3g
sim card
to my o2 online payg service. Reason i ask is i have just brought a o2
3g sim on ebay
http://cgi.ebay.co.uk/ws/eBayISAPI.d...MEWN%3AIT&rd=1
But not even sure it will be any use to me, dont know if it's for
contract or payg service,
only know it is brand new.
Can i just call o2 up and say i have lost my sim and ask them to
transfer the service to this 3g sim?

Thanks for your help.

Steve.