For Sonic Fusion, that option will, indeed, switch your existing AT&T phone line, phone number, and broadband service over to Sonic.net. AT&T is automatically notified of the "line loss" and will discontinue your phone service and send you a final billing for your service.
When I switched, I had broadband service with a different provider (other than AT&T), and I need to call them to notify them of the loss.
It never hurts to check with your previous provider to ensure they've gotten the word, but whatever you do, DON'T call them before you order service from Sonic. If you should call your existing provider to cancel before you make your order with Sonic, it will result in loss of your service for some time while it's all straightened out.