Hi Downtown. Because the modem has to be provisioned for it to work, you would need to contact us if you decide to use the old modem. If you have Gigablast, the old modem will only work it if is a DOCSIS 3.1 Modem. We don't send any signals to the router so you can change it out, however, you will need to reset the modem before connecting the new router for it to work. If you need further help, please send an email to cox.help@cox.com and include your complete address and name on the account. - Lisa, Cox Support Forums Moderator