You can't do it online. But an employee here can help, or you can call in again.
I've asked a Comcast employee to help you. You should expect a reply in this thread.
Hello Smgh1. I can help you change your service. Sometimes, changing the service order dramatically can cause a shift in the appointment time / availability. Please send me a private message and include your full name, service address, and account number so I can assist you.