In my experience, release/request by itself isn't generally enough if you're moving from one network to another.
My procedure to clear out the current network configuration on, say, lan0 is (I have a 'fluship' script to do this) is:
dhcpmon -t
route -f
ifconfig lan0 down
ifconfig lan0 delete
arp -f
Then (assuming I'm connected to the new network), I run mptn\bin\setup.cmd, which will start the DHCP client on lan0 with a fresh configuration.
This works well 99% of the time. Very rarely, something of the old configuration seems to get stuck to the point that only a reboot will clear it.