Yeah, I did it a long time ago. The actual details are a little fuzzy, but I seem to remember . . .
I had to call earthlink to activate my accounts (e-mail, etc.) on the Earthlink end.
Then I was given a special number to call to talk to Comcast. That is, not the regular Comcast number but a Comcast number that was only for Earthlink members to call. During that call, I was told to activate my web browser (remember, I still had the comcast cable modem) and direct it to a certain url. I hit a few links, and it asks me if I want to activate Earthlink service or some such. (yes) Then the modem was re-provisioned, and I started getting billed by Comcast for "earthlink" service. Done. -Dave