CM2000 and Xfinity 2Gbps Service

I have a Netgear CM2000 modem. Yesterday I tried to upgrade my Xfinity service from their Gigabit Extra service (1.2 Gbps down and 40 Mbps up) to their 2 Gbps service (which includes 400 Mbps up). I finally decided to pull the trigger since they now, finally, support customer owned equipment on their 2 Gbps service. Complete failure. While the Xfinity site shows they support the CM2050v for their plans the CM2000 is listed but not recommended. Once Xfinity switched me to 2Gbps the modem just cycled over and over - never got a connection. 4 tech support people later, I finally threw in the towel and had them return my service to the Gigabit Extra service and everything works well again. I would like to upgrade without buying a new modem - any ideas? I did all the usual things - unplugged/replugged modem and router numerous times, etc.