The process is ->
SwapMySim Procedure.
If a person upgrades to another phone and the new phone takes another sized Sim Card then this procedure moves the Tariff and Mobile number from the original SIM card to the new one. If on PayAsYouGo any credit will move across too.
This is the same if a new SIM card is required if a person feels their original one may be at fault.
Any O2 HighStreet Store can perform the swap in store (if on Contract please take some Photo I.D just incase), or a person can take the SIM card home or request one via O2 Customer Services. 202 for Contract. 4445 for PAYG. 8002 for Business ->
http://www.o2.co.uk/contactusSim Swap link ->
http://swapmysim.o2.co.uk Or text SWAP to 20220
Be sure to ask for an Account Specific Blank SIM card as Contract and Pay As You Go versions hold different information. And please take some Photo ID for Contract Sim Cards just incase your asked.
The Service will stop on the original SIM card which indicates the swap has started. Turning the phone Off and On , or if applicable switching airplane mode On and Off ,with the new SIM card in every 20 minutes or so will typically help the connection to settle.