When switching to a new supplier, the last thing you want is to have a contract to pay both your old and your new suppliers. Don't worry!
The new supplier is responsible for letting the old supplier know to cancel the contact and is done automatically.
When a new supplier takes your suppliers they are taking it from the old supplier. Whilst it looks very seamless it there are agreements happening in the background to agree on your annual usage figures, produce a final bill, and close your contract.