Here is the Official Comcast Customer Support answer for downgrading your service:
Change or Upgrade Your Service
To upgrade your services, please log into My Account and shop for upgrades. Or if you’d prefer, please call us at the number below and we can help.
If you’d like to downgrade or change your package, we are not able to do that online yet. However, our customer service agents are happy to assist you.
Please call us at 1-800-XFINITY (1-800-934-6489).
So there you have it. A Billion dollar cable and ISP company is INCAPABLE of providing downgrade information or availability ONLINE but UPGRADING IS AVAILABLE! I’m sure it’s just too technically taxing for them and has NOTHING to do with making their customers frustrated with monopolistic BAD SERVICE.