howto migrate from esktop to server

There isn't a big difference between server and desktop versions. The first one provides a graphical desktop environment by default, while the second one gives you the option to set up a LAMP server.
Packages are the same, so if you want to set up a server installation, you can simply turn off (or remove, if you prefer) your X server and install the packages you need. Ubuntu lacks a vritual package (or a task) which provides an easy installation of a server environment starting from a desktop version, anyway it is not a very difficult task to accomplish.

You basically need to uninstall the ubuntu-desktop package and all of its dependencies except things like ubuntu-minimal or ubuntu-standard or packages which want to remove them.

If you are unfamiliar with terminal based managment then you can leave the desktop installed and just change the configuration so it does not start at boot.
I am sorry I am not on my Ubuntu system at the moment so I cannot check the settings.
 Basicly you would want to to init 3 and not init 5.
Look at your inittab file. There should be a line where boot option is set to init 5. Change this to init 3 and reboot. That should boot to console.
Now if you want to log in and work jsut type :
 init5
and it should switch to runlevel 5 and the desktop. When your done hit
ctrl+alt+F1 and type
init3
to switch back.

Except ubuntu uses runlevel 2 by default for multiuser + console. So first you'd have to define your own runlevel by renaming the symlinks in /etc/rc3.d (for example) there's a file that describes what to rename them too in /etc/rc3.d/README

Thanks.
It pleases me