To order EarthLink Cable over Comcast lines, call 1-866-570-8333 and they will initiate the switch. The first step is to speak with the EL rep and check availability. They will then set you up with your first email address and then transfer you to Comcast so they can make the switch on their end. Comcast takes about 10-15 minutes to do their part and they will either call you back or you can stay on hold for the next step. They will send you to the online provisioning page to reprovision you with EarthLink as your ISP. Your domain will then switch from Comcast to EarthLink.
After you have switched, you can proceed to setup more email accounts from the EarthLink accounts page