DHCP Release or Disable BootP?

A friend of mine admins a network with two subnets. A number of times a week, certain users with laptops have to travel to the other subnet. When they logon they continue with the last ip and subnet config they were previously using. My friend wants them to be able to travel between subnets, use DHCP, and not have to reconfigure TCP/IP each time. One option that may be lacking from my friends list is turning off BootP forwarding due to a controlling boss. Is there a way to have a machine release it’s IP at each shutdown? Is there a better solution to this problem?