059 -- If you are in the Greater Chicago area and are a TV only customer we can submit a ticket to request that type of change to your billing due date. However, an Xfinity Double, Triple or Quad Play customers and TV only customers in other areas do not qualify for this change.
I just ran into the same brick wall. No matter how hard I pressed them, they flatly refused to change it and refused to explain why this wasn't possible. Since there are plenty of other large companies out there with less techincal ability that do allow this, I can only assume that this choice is deliberate on Comcast's part. They know that if customers could change their billing dates, Comcast wouln't be able to charge as much in late fees, so they simply won't allow changes.
And Comcast wonders why their public image is so extraordinarily bad... It's things EXACTLY like this that get them the reputation they "enjoy". Absolutely ridiculous. Refusing to change billing dates should be illegal, as far as I'm concerned.