Internet performance immediately degraded after changing plan despite keeping the same internet package

Recently made some updates to our Xfinity plan to remove some services and packages we weren't using. I stressed to the agent I was working with to develop the new plan that it was imperative that our internet service stay exactly the same as it was on our old plan. The agent assured me that my internet service would remain unchanged.

After closely reviewing the order sheet, I clicked the confirmation button at the bottom. My modem, a Motorola MT7711, immediately reset, which I expected as we've been through this before. When it came back online, I noticed that the downstream connection indicator light was solid green, indicating that it is only bonding with one downstream channel. It should have been (and was prior to the plan change) solid blue denoting a bond with two or more channels per the modem's user manual. Subsequently, when I tried to play Xbox, watch TV via Xfinity Stream and use my phone on wifi I noticed severe slowing and packet loss, and speed tests showed download speed that are less than a quarter of what we pay for. Upload performance has remained at acceptable levels throughout.

Investigating the issue in my modem's admin page revealed downstream power levels and SNR values below acceptable ranges, along with a swath of T3 timeout errors. This is all after my internet had been performing very well for a long time leading up to the plan change. I did not touch a single piece of physical equipment before or after the plan change. The modem, coaxial cables, and wall jacks are all the same. No splitters are present anywhere in the signal chain. What was done to my internet service? How is it possible that clicking "agree" on a new plan suddenly made all my equipment go bad?