I have a Comcast cable modem serving DHCP. Originally, it was using range 10-255, which conflicted with a DNS server managing IPs 10-30. I've fixed the range used on the modem, but how to I force all clients using conflicting IPs in the 10-30 range to release and renew? There are several dozen machines (a mixture of laptops, desktops, and voip phones), so it would be very labor intensive to manually visit each machine.
You cannot force them to renew using any software method. If you actually have conflicting addresses then you probably couldn't even reach the systems over the network, assuming you are only using TCP/IP and you don't have IPX or something else setup.
About the fastest way to force everything to attempt a renew would be to go to your power mains box, and kill the power for a couple minutes. Pretty much every OS will attempt a renew when they boot/reboot.