Yup... just move the wires over and you should be good to go. If you're not online when you do then hook up your PC to the modem via Ethernet and reboot the modem. When it comes up and you open a browser you should get the activation page where you'll login and click a couple of things and that will activate the modem.
I received a replacement modem for faster speed. Can I just replace the NEW MODEM with the existing wires I already have. ??
Connect it to the coax line and power it up. Let it fully sync up with the system ("DS, "US", and the "Online" lights are steadily lit). Connect a computer to it and open up a browser. You *may* see their "Walled Garden" modem self registration page. If so, follow the screen prompts. Have your account number handy.
If not, try calling 1-855-652-3446. That is their device activation line.
If that doesn't work for you you can call 1-800-comcast and tell a rep that you would like to have a new modem provisioned for service. Have the RF / HFC MAC address and the serial number of it handy when you call.
FWIW, something else is going on with the device synching up with the CMTS / system. If there are no RF signal / connection quality / block synch issues hardware malfunctions, the "Online" light should be lit steadily regardless of whether or not a modem has yet been activated / provisioned for service