Setting and Interface for DHCP
If you want to change an interface to use DHCP, edit /etc/network/interfaces making sure you remove the static config
iface eth0 inet static
iface eth0 inet dhcp
The reverse can be done to change from DHCP to static.
Adding Alternative DNS server
Since DNS servers are handled by the DHCP, I wanted to add an alternative DNS server. Editting the dhclient.conf lets you do this and adding the following the lines to the file.
prepend domain-name-servers 127.0.0.1
or use supersede to replace them.
supersede domain-name-servers 127.0.0.1
Now the your Ubuntu server can run with DHCP with alternative DNS servers.
Use /etc/init.d/networking restart to apply the changes for the DHCP and DNS right away.