I know with my former cable modem anytime I changed the device connected to it, I had to reboot the modem.
With the DSL modems currently shipping, if the DSL service is PPPoE, the modem could be running the PPPoE session. In which case, there would be no need to reboot the modem after an equipment change. -- Norman ~Oh Lord, why have you come ~To Konnyu, with the Lion and the Drum