Swapping out personal modem, that is already consigned on Xfinity plan, with a new personal modem.

I have an older modem that has its MAC address already registered to Xfinity / Comcast.  The Modem needs to be replaced with a newer personal modem.  What am I supposed to do to swap out the old modem and register the new modem?  I do not want to change my existing plan.  Who do I contact?  What is the registration process?  How do I unregister the older modem?  Thank you.

New modem will be an ARRIS Surfboard S33 DOCSIS 3.1