On Ubuntu when I move my laptop from one subnet to another (desk to conference room for example) the dhcp ip doesn't reload. Previously I would disable the adapter then re-enable it but that took a bit of time. I found that you can use dhclient to do the trick from a shell:
sudo dhclient -r to release ip.
sudo dhclient -1 to renew ip. (that is a -one not the letter 'l').