How to migrate(change) from Ubuntu desktop to Ubuntu Server?

Open your terminal and type:

sudo apt-get install tasksel

Then type sudo tasksel in terminal

It will open in terminal with options & there you can choose Ubuntu Server with space-bar and then continue. It will download and install Ubuntu Server tools.

Hope that helps.

sudo apt-get install tasksel
sudo tasksel remove ubuntu-desktop
sudo tasksel install server
sudo apt-get update
sudo apt-get install linux-server linux-image-server
sudo apt-get purge lightdm

These commands can help you to competly convert an Ubuntu Desktop to Ubuntu Server. It work on Ubuntu 14.04 ;D