Usually it's not necessary to reboot a client when DHCP changes your IP
address, but usually this change also never happens...

In our environment we use DHCP for NT servers and workstations without any
problems. The client gets an IP address lease from the DHCP server (with a
validity of 1 week in our case). As there's no need for the DHCP server to
change the address of a client when the lease has expired, we never get such
changes when the client is online. Even when the lease is expired and much
time has already passed by, the client usually gets the old address again as
the MAC address of the network interface card is remembered. When you change
the NIC, you likely will get a new IP address, but in this case a reboot
should have happened.

Best regards,
Michael