There's no need to contact your old retailer when you decide to switch to a new one.
The new retailer will commence the switch process on your behalf, and the old retailer will automatically be notified.
It's possible they might make a retention offer for you when they discover you're leaving. If they do, make sure you get the details in writing and send them to Bill Hero. We'll help you see if that retention offer actually represents good value for you or not.