Changing modem/router on the fly

The Netgear C6300 is compatible but it depends on your speed tier. It's good for speeds up to 369 Mbps down;

https://www.xfinity.com/support/devices#details

Perhaps I  did not make myself clear. I know what speeds the Netgear will support. I have much less than that. I am on the cheap end at 25mbs. I was wondering if there was a way to unplug the Comcast modem, plug in my Netgear to see if it works. It was given to me and I can save $14.00 a month if the Netgear will work. 

Hopefully I have made this more understandable. 

Plug it in and call 1800comcast. Ask the rep to activate / provision it for service.

In my cable plant, you can activate the modem when the PC auto launches an activation page. Just insert the info they want to know and it will reboot. Then you will be online with the new device. If no page or if no luck using self activation, you can call Comcast at 1800-266-2278. Once that is done, power off the modem, and connect it to the router, then power cycle the modem.

thanks to all. appreciate the help.