Unable to upgrade my internet plan

On the Xfinity site, I see an offer to switch from 800 mbps to 1 gbps for only $30/month, about $25/month cheaper than my current plan. When I add the plan to my cart, I get an error saying I have to contact Xfinity.

I livechatted with an Xfinity agent and they ran into the same problem. Then I called Xfinity and the phone agent ran into the same problem.

Is this a problem with my account, or is it something with Xfinity, maybe the Microsoft Azure outage? And what's the best way to get my account upgraded?