If you are still using the same router, your external IP address may stay the same, as I believe Comcast assigns this based on the MAC address of the router.
However, if you have your router disconnected from Comcast for period of time you would likely get a new dynamic external IP assigned (would have to check with Comcast as to how long you need to disconnect the router for to get a new IP).
Ipconfig/release would affect your LAN IP only, and wouldn't change your external IP as you are behind a router. If your external IP does not change after disconnecting the router you may be able to clone the MAC address to get a new external IP.
Here is a link that may help you ~http://www.ehow.com/how_5923342_obtain- ... cable.html