The Xfinity website contains information on upgrading equipment and updating firmware. A relevant page can be found by searching "upgrade your device" on the website and clicking on Upgrade Your Wireless Network Equipment on the results page.
The page has details on upgrades for both routers and modems, as well as information on how to update these devices' firmware. It also lists models that receive automatic firmware updates from Comcast.
In many situations, a device upgrade may not be required, but it may enable new network features. If a customer has recently updated his speed tier and his current modem doesn't support the new connection speed, a modem upgrade may be required.