I have changed it there and then. They usaly don't need notice,They just tell you the new charges and it's all done over the phone in a few minuets (that's not including the put on hold bit and the clasical tune sample played 50 times before a real person speaks to you!)
It would be best to tell them a few days before you get new car as they
need to know the details and new number and you may have to pay
extra if value of car is much more than your present one and also to be covered when you collect car you can tell them when you want cover to
start.
Remember not all insurers are open of a weekend, so if you're picking up your new Jamjar on Saturday, best to call your insurer Thursday or Friday.
True, those that are closed may be fairly rubbish, but best to be prepared rather than be stood on Arthur Daley's car yard listening to a recorded message saying 'bugger off, call again Monday'