Illness Benefit - Change of Medical Condition

What is the correct procedure for changing the medical condition while receiving illness benefit.
The situation is that a relative has been receiving illness benefit for a specific illness , On further investigation his GP has now diganosed a more serious condition which will prevent him from working for a prolonged period. Does the GP just change the condition on his weekly certificates or does he need to make a new claim?

The GP certifies the medical certificate with whatever illness he sees fit to put down. No need for new claim, although a note explaining the sitaution might help for future reviews (i.e. a prolonged illness might mean going onto Invalidity Pension down the line)

Great contribution there alright

The doctor needs to change the illness on the cert, thats all. No new claim is needed.

Thats all I need to know. Thanks for the replies.